Magnetic tape and magnetic tape cartridge

ABSTRACT

A magnetic tape comprising a non-magnetic support, a magnetic layer containing magnetic powder which is formed on one side of the non-magnetic support, a primer layer containing non-magnetic powder which is formed between the non-magnetic support and the magnetic layer, and a backcoat layer containing non-magnetic powder which is formed on the other side of the non-magnetic support, wherein the magnetic layer contains the magnetic powder which comprises plate, granular or ellipsoidal magnetic particles with a particle diameter of 5 to 50 nm, and has a thickness of 0.09 μm or less, and wherein at least one of the primer layer and the backcoat layer contains non-magnetic plate particles with a particle diameter of 10 to 100 nm.

TECHNICAL FIELD

The present invention relates to a coating type magnetic tape with ahigh recording density.

BACKGROUND ART

Magnetic tapes have found various applications in audio tapes,videotapes, computer tapes, etc. In particular, in the field of magnetictapes for data-backup (or backup tapes), tapes with memory capacities ofseveral tens to 100 GB per reel are commercialized in association withincreased capacities of hard discs for back-up. In future, a backup tapewith a capacity of 1 TB or more will be proposed, and it isindispensable for such a backup tape to have a higher recording density.

In the production of a magnetic tape capable of meeting such a demandfor higher recording density, advanced techniques are required forproduction of very fine magnetic powder, highly dense dispersion of suchmagnetic powder in a coating layer, smoothing of such a coating layer,and formation of a thinner magnetic layer.

To increase the recording density, recording signals with shorterwavelength and tracks with shorter pitches are required, and there hasbeen emerged a system using servo tracks so that a reproduction head cancorrectly trace the tracks.

To meet the main demand for recording of signals with shorterwavelength, magnetic powder for use in magnetic tape have been improvedto have more and more fine particle size and also improved in magneticcharacteristics. In the field of the existing data backup tape, magneticpowders of ferromagnetic iron oxide, Co-modified ferromagnetic ironoxide, chromium oxide and the like, used in audio systems and householdvideo tapes have been dominantly used. Presently, needle-shape metallicmagnetic powder having a particle size of 100 nm or so has beenproposed. On the other hand, to prevent a decrease in output due todemagnetization in recording signals with shorter wavelengths, backuptapes with higher coercive forces have been vigorously developed year byyear. As a result of such developments, backup tapes with coerciveforces of about 198.9 kA/m have been accomplished by the use ofiron-cobalt alloys (JP-A-3-49026, JP-A-5-234064, JP-A-6-25702,JP-A-6-139553, etc.).

In the meantime, the media-producing techniques have been significantlyadvanced by the development of binder resins having a variety offunctional groups, the improvement of the dispersing technique for theabove magnetic powder, and further the improvement of the technique ofcalendering after the coating step. These improvements have markedlyimproved the surface smoothness of magnetic layers and contributedgreatly to an increase in output of signals with shorter wavelengths(for example, JP-B-64-1297, JP-B-7-60504, JP-A-4-19815, etc.).

In association with the recent high density recording, the recordingwavelength becomes shorter and shorter. Therefore, in case where thethickness of a magnetic layer is large, the saturation magnetization andthe coercive force of conventional magnetic powder are insufficientwithin the shortest recording wavelength region, so that the reproducingoutput decreases to a fraction thereof. Further, because the recordingwavelength is very short, self demagnetization loss and thickness lossdue to the thickness of a magnetic layer give adverse influences on theresolution, although such demagnetization loss and thickness loss whichoccur when recorded signals are reproduced have not arisen so seriousproblem so far. This problem can not be overcome by the aboveimprovement of the magnetic characteristics of magnetic powder and theimprovement of the surfaces of magnetic layers by the medium-producingtechnique. Under such circumstances, it is proposed that the thicknessof a magnetic layer should be reduced.

Generally, it is said that the effective thickness of a magnetic layeris about one third of the shortest recording wavelength used in thesystem. For example, the thickness of a magnetic layer is required to beabout 0.1 μm when the shortest recording wavelength is 0.3 μm. With thetrend of compacting a cassette (or a cartridge) for holding tape, awhole of magnetic tape is needed to be thinner so as to increase therecording capacity per volume. To meet such a demand, it is consequentlyneeded to form a thinner magnetic layer. Further, to increase therecording density, a magnetic flux for writing which a magnetic headgenerates should have a very small area. In this connection, compactingof the magnetic head results in a smaller amount of magnetic fluxgenerated thereby. In order for the above very small magnetic flux tocause a perfect magnetic inversion, it is necessary that a magneticlayer should be formed with a thinner thickness.

However, there arise other problems in the formation of a thinnermagnetic layer. That is, when the thickness of a magnetic layer isreduced, the surface roughness of a non-magnetic support gives anadverse influence on the surface of the magnetic layer, so that thesurface smoothness of the magnetic layer degrades. When a singlemagnetic layer is formed with a thin thickness, the solid content in apaint for magnetic layer should be decreased, or the amount of the paintto be applied should be decreased. However, the defects of coating arenot eliminated and the filling of magnetic powder is not improved bythese methods, which results in poor film strength. To overcome thisproblem, the following concurrent coating-and-laminating method isproposed: that is, in case where a thinner magnetic layer is formed byan improved medium-producing technique, a primer layer is providedbetween a non-magnetic support and a magnetic layer, and the uppermagnetic layer is applied on the primer layer which is still in a wetstate (JP-A-63-187418, JP-A-63-191315, JP-A-5-73883, JP-A-5-217148,JP-A-5-298653, etc.).

When the recording density in the tape-widthwise direction is increasedby narrowing the width of the recording tracks, magnetic flux leakingfrom the magnetic tape is decreased. Therefore, it is needed that MRheads using magneto-resistance elements, which can achieve high outputeven when the magnetic fluxes are very small, are used for reproducingheads.

Examples of a magnetic tape which can correspond to MR heads aredisclosed in JP-A-11-238225, JP-A-2000-40217 and JP-A-2000-40218. In themagnetic recording media described in these publications, skewness ofoutputs from the MR heads is prevented by controlling the magneticfluxes from the magnetic recording media (a product of a residualmagnetic flux density and the thickness of a medium) to a specific valueor less, or the thermal asperity of the MR heads is reduced bycontrolling the dents on the surface of the magnetic layer to aspecified value or less.

When the width of the recording tracks is decreased, the reproducingoutput lowers due to off-track. To avoid such a problem, track servocontrol is needed. As types of such track servo control, there are anoptical servo system (JP-A-11-213384, JP-A-11-339254 andJP-A-2000-293836) and a magnetic servo system. In either of thesesystems, it is necessary that track servo control is performed on amagnetic tape which is drawn out from a magnetic tape cartridge (or acassette tape) of single reel type which houses only one reel forwinding the magnetic tape, in a box-shaped casing body. The reason forusing a single reel type cartridge is that, when the tape-running speedis increased (for example, 2.5 m/second or higher), a tape can not bereliably run in a two-reel type cartridge which has two reels fordrawing out the tape and for winding the same. The two-reel typecartridge has other problems in that the dimensions of the cartridgebecome larger and that the memory capacity per volume becomes smaller.

As mentioned above, there are two types of track servo systems, i.e.,the magnetic servo system and the optical servo system. In the formertrack servo system, servo track bands are formed on a magnetic layer bymagnetical recording, and servo tracking is performed by magneticallyreading such servo track bands. In the latter optical servo type, servotrack bands each consisting of an array of pits are formed on a backcoatlayer by laser irradiation or the like, and servo tracking is performedby optically reading such servo track bands. Other than these types,there is such magnetic serve system in which magnetic servo signals arerecorded on a magnetized backcoat layer (for example, JP-A-11-126327).Further, in other optical servo system, optical servo signals arerecorded on a backcoat layer, using a material capable of absorbinglight or the like (for example, JP-A-11-126328).

In general, recording tracks are written in the tape lengthwisedirection in a linear recording type computer tape, and the width of thetracks of a reproducing head (reproducing track width) is set at a valuefairly smaller than the recording track width: for example, therecording track width is about 28 μm, while the reproducing track widthis about 12 μm; or the recording track width is about 24 μm, while thereproducing track width is about 12 μm. By doing so, the off-trackmargin is increased, and thus, a decrease in reproducing output ishardly caused, even when the position of the magnetic tape is dislocatedby about 3 μm (dislocation due to edge weave on the tape or a change insize due to changes in temperature and/or humidity) or when there isabout 3 μm of dislocation of tracks between each of units. Because ofsuch a sufficient off-track margin, it is not needed to pay a carefulattention on the edge weave of the magnetic tape or the widthwisedimensional stability thereof against changes in temperature and/orhumidity.

Problems to be Solved by the Invention:

However, the improvement of the magnetic powder and themedium-fabricating techniques have now reached the uppermost limit.Particularly in the improvement of the magnetic powder of needleparticle type, the particle size thereof is reduced to about 100 nm asthe smallest in view of practical use. This is because, when theparticle size is smaller than about 100 nm, the specific surface area ofthe magnetic powder markedly increases, and the saturation magnetizationlowers, and also, it becomes very difficult to disperse such magneticpowder in a binder resin.

The technical innovation of magnetic heads has made it possible torecord signals on media having high coercive forces. Particularly in thelengthwise recording system, it is desirable that the coercive force ofa magnetic layer should be as high as possible to an extent that theerasing of the recorded signals by a magnetic head is possible, so as toprevent a decrease in output because of demagnetization by recording andreproducing. Therefore, the practical and most effective method forimproving the recording density of a magnetic recording medium is toincrease the coercive force of a magnetic recording medium.

To lessen the influence of a decrease in output due to demagnetizationby recording and reproducing which is the essential problem of thelengthwise recording system, it is effective to further decrease thethickness of a magnetic layer. However, there is a limit in thethickness of a magnetic layer, as long as the above magnetic powderhaving a needle particle size of about 100 nm is used. The needleparticles are generally arrayed such that the needle-pointed directioncan be in parallel to the in-plane direction of a medium, because of thelengthwise orientation of the needle particles. However, some of theneedle particles are arrayed vertically to the plane of the medium,since there is a distribution in the dispersion of the particles.Because of such needle particles, the surface of the medium becomesuneven to increase the level of noises. This problem becomes moreserious as the thickness of the magnetic layer is more and more thinner.

In case where a magnetic layer is formed with a thinner thickness, it isneeded to dilute a paint for magnetic coating with a large amount of anorganic solvent. The conventional needle particle type magnetic powdertends to agglomerate paints for magnetic coating. In addition, the largeamount of the organic solvent is evaporated off when the magnetic layeris dried, which degrades the orientation of the magnetic powder. Thus,the lengthwise recording tape medium becomes poor in the orientation,and it becomes difficult to obtain desired electromagnetic conversingcharacteristics therefrom because of degradation of the orientation andthe surface of the magnetic layer, even though the magnetic layer isformed thinner. In spite of the known fact that the use of a thinnermagnetic layer is effective to improve the recording characteristics inthe lengthwise recording system, it is still difficult to obtain acoating type magnetic recording medium which comprises a magnetic layerwith a far reduced thickness, insofar as the conventional needleparticle type magnetic powder is used.

Among several kinds of magnetic powder which hitherto have beenproposed, barium ferrite magnetic powder is known which comprises plateparticles and has a particle size (or a particle diameter) of about 50nm (for example, JP-B-60-50323, JP-B-6-18062, etc.). This barium ferritemagnetic powder is more suitable for a thin layer coating type magneticrecording medium, than the needle particle type magnetic powder, becauseof the particle shape and particle size (or particle diameter) of thebarium ferrite magnetic powder. However, since the barium ferritemagnetic powder is an oxide, its saturation magnetization is about 70A.m²/kg (70 emu/g) at most, and therefore, it is theoreticallyimpossible to obtain saturation magnetization of 100 A.m²/kg (100 emu/g)or more which needle particle type metal or alloy magnetic powder canshow. The use of the barium ferrite magnetic powder makes it possible toobtain a coating type magnetic recording medium having a thin magneticlayer, but is unsuitable for a high density magnetic recording mediumfor use in a conventional system which uses a magnetic induction typemagnetic head as a reproducing head, because the output is low due tolow magnetic flux density. For this reason, the foregoing needleparticle type magnetic powder has been dominantly used as the magneticpowder for high density magnetic recording media. However, in a systemin which a highly responsive magnetoresistance type magnetic head isused for a reproducing head, the above defects of the barium ferritemagnetic powder can be eliminated. Therefore, it is possible to use thebarium ferrite magnetic powder in a magnetic recording medium accordingto the present invention.

As is understood from the above description, in the formation of amagnetic layer with a thin thickness which is one of the effectivemethods for improving the recording density of a magnetic recordingmedium, it is very important to maintain the coercive force and thesaturation magnetization of magnetic powder at values as high aspossible and simultaneously to reduce the particle size thereof. Toachieve this subject matter, the present inventors, firstly, have paidtheir attentions on the magnetic characteristics of the conventionalmagnetic powder and found that a theoretical limit is present inachieving a higher coercive force since the conventional needle particletype magnetic powder gains a coercive force based on the shapeanisotropy induced by its needle particles. In other words, in the shapeanisotropy, the magnitude of the magnetic anisotropy is expressed by2πIs (wherein ‘Is’ represents saturation magnetization), and isproportional to the saturation magnetization. Therefore, the coerciveforce of the needle particle type magnetic powder based on the shapeanisotropy becomes larger in proportion to an increase in saturationmagnetization.

As is well known from the Slater-Pauling curve, the saturationmagnetization of a metal or an alloy, for example, a Fe—Co alloy, showsa maximal value at the ratio of Fe/Co of about 70/30. Therefore, thecoercive force of this alloy shows a maximal value at the abovecomposition ratio. Needle particle type magnetic powder of Fe—Co alloyin the ratio about 70/30 has already been practically used. However, ashas been described above, whenever the needle particle type magneticpowder is used, the coercive force thereof is theoretically limited toabout 198.9 kA/m at most at the present, and it is difficult to achievea higher coercive force under the present circumstances. Therefore, theuse of such needle particle type magnetic powder is unsuitable for athin layer coating type magnetic recording medium.

The magnitude of magnetic anisotropy in the shape anisotropy isexpressed by 2πIs as mentioned above, and the coefficient is representedby 2π when the an aspect of magnetic powder (the particle length/theparticle diameter) is not smaller than about 5. When the an aspect issmaller than 5, the coefficient rapidly becomes smaller. When theparticle shape is spherical, the anisotropy thereof vanishes. In otherwords, in the state of the art, insofar as a magnetic material such as aFe metal, a Fe—Co alloy or the like is used as magnetic powder, theparticle shape of the magnetic powder inevitably and theoreticallyresults in the shape of needle.

Also, in the prior art, a primer layer with a thickness of about 2.0 μmis formed on an non-magnetic support, and a magnetic layer with athickness of about 0.15 to about 0.2 μm is formed on the primer layer,in order to improve the characteristics of recording/reproducing ofsignals with short wavelength. To further improve the recording density,the thickness of the magnetic layer is preferably 0.01 μm to 0.09 μm,more preferably 0.06 μm or less, particularly 0.04 μm or less. Thethickness of the primer layer is preferably 0.2 μm or more, morepreferably 0.3 μm or more. The thickness of the primer layer ispreferably 1.0 μm or less, more preferably 0.8 μm or less, particularly0.5 μm or less. In this regard, preferably the primer layer isnon-magnetic. If the primer layer is magnetic, the record on themagnetic layer formed on the primer layer is disturbed by themagnetically recorded signals on the primer layer, or the magneticallyrecorded signals on the primer layer may skew reproduced signals.

As mentioned above, for a higher recording capacity of a computer tape,the pitches between each of the recording tracks become narrower andnarrower, and the recording capacity per one reel of tape is reaching 1TB. To exceed 1 TB, the width of recording tracks is required to be 12μm or less on calculation. The width of reproducing tracks is determinedtaken into account the resultant output and the off-track margin. Thevalue of (the recording track width−the reproducing track width) isexpected to decrease to 5 μm or less from the current 12 μm. If so, thewidth of the off-track margin is severely restricted. For example, oncondition that the recording track width is 12 μm and that thereproducing track width is 10 μm, the value of (the recording trackwidth−the reproducing track width) is 2 μm as the sum of both sidemargins and 1 μm as one side margin. Thus, the off-track margins betweenthe magnetic tape and the apparatus are as narrow as 0.5 μm,respectively. In such a case, the amount of edge weave is preferablysmaller than 0.8 μm, more preferably smaller than 0.6 μm, and zero asthe best.

To correctly trace tracks in correspondence with track pitches whichhave become narrower and narrower, it is necessary that the spacingdimensions between the tape edge and data tracks, the tape edge andservo tracks and the servo track and data tracks should be kept constantagainst changes in temperature and humidity, in other words, that thetemperature and humidity expansion coefficients in the tape widthwisedirection should be small. The temperature expansion coefficient in thetape widthwise direction is preferably 0 to 8×10⁻⁶/° C., more preferably0 to 6×10⁻⁶/° C., and the humidity expansion coefficient in the tapewidthwise direction is preferably 0 to 10×10⁻⁶/% RH, more preferably 0to 8×10⁻⁶/% RH, and zero as the best.

As the track pitches become narrower, the non-uniformity of thethickness of a magnetic coating layer gives a more serious influence onoutput and causes a larger fluctuation in output. This leads to a highererror rate. In case of a magnetic medium in which a non-magnetic primerlayer and a magnetic layer are applied on a non-magnetic support whilethe non-magnetic primer layer is being wet, a disorder is likely tooccur at the interface between the non-magnetic primer layer and themagnetic layer in the coating step, the orientation step in a magneticfield, and the drying step. Such a disorder at the interface becomes aserious factor to vary the thickness of the magnetic layer. The methodsof decreasing variation in the thickness of magnetic layers aredisclosed in JP-A-5-73883, JP-A-10-69635, JP-A-2001-134919,JP-A-2001-256633, etc. Some of these publications describe that amagnetic layer is applied after drying a non-magnetic primer layer.Other publications describe that a non-magnetic coating paint and amagnetic coating paint are made thixotorpic so that their thixotropiescould be close to each other, or that needle particle type filler iscontained in a non-magnetic coating paint. However, the method ofapplying the magnetic layer after drying the non-magnetic primer layeris difficult to regulate the thickness of the magnetic layer to 0.09 μmor less, preferably 0.06 μm or less, more preferably 0.04 μm or less.The method of preparing the non-magnetic coating paint and the magneticcoating paint thixotropic so that their thixotropies can be close toeach other, and the method of containing the needle particle type fillerin the non-magnetic coating paint are found to have the followingproblems. In case where the thickness of the magnetic layer is 1 μm orless, the ratio of the variation amount of the thickness of the coatinglayer (Δd) to the thickness of the coating layer (d) is 0.5 or less, andthe ratio of the standard deviation of the amount of variation of thethickness of the coating layer (STDEVΔd) to the thickness of the coatinglayer (d) is 0.2 or less. In case where the thickness of the magneticlayer is 0.01 to 0.3 μm, the ratio of the standard deviation of theamount of variation of the thickness of the coating layer (STDEVΔd) tothe thickness of the coating layer (d) is 0.5 or less. As a magneticlayer is formed far thinner, the variation rate (%) (=the amount ofvariation in the thickness of the coating layer (Δd)/the thickness ofthe coating layer (d)×100) becomes far larger, even if the variationamount of the thickness is the same. Therefore, the uniformity in thethickness of a magnetic layer should be achieved in order to realize amagnetic tape comprising a magnetic layer with a thickness of 0.09 μm orless, preferably 0.06 μm or less, more preferably 0.04 μm or less as inthe present invention, as will be described later.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a breakthroughtechnique for overcoming the foregoing problems, and thus to provide amagnetic tape having a high recording density capable of correspondingto a recording capacity of 1 TB or more per one reel of tape, and amagnetic tape cartridge comprising the same.

To achieve the above object, the characteristics of magnetic powdernecessary for markedly increasing the recording density of a coatingtype magnetic recording medium which comprises a thin magnetic layer aredescribed in the following items (1) to (5), and raw materials and aproduction process suitable for such a magnetic recording medium havebeen researched and developed under these basic guides (1) to (5).

(1) Magnetic powder having a coercive force as high as possible, to anextent that the deletion of records with a magnetic head is possible.

(2) Magnetic powder showing as large saturation magnetization aspossible, and comprising, as main components, elements which areabundantly present as resources.

(3) Magnetic powder having a particle shape of a plate, grain orellipsoid which has good filling properties.

(4) Magnetic powder comprising as fine particles as possible, to anextent that the magnetic powder can maintain saturation magnetization.

(5) Magnetic powder having an uniaxially magnetic anisotropy in whichthe magnetization is easy in a single direction.

The present inventors have researched magnetic powder which can satisfyall the above properties, and found that magnetic powder having aparticle diameter of 5 to 50 nm (preferably 5 to 30 nm, more preferably5 to 25 nm) and a particle shape of a plate, grain or ellipsoid cansatisfy these properties and can provide an excellent recording mediumwith a high density. Examples of such magnetic powder are magneticpowder of barium ferrite and magnetic powder of rare earth-iron-borontype. It is demonstrated that a magnetic recording medium using the rareearth-iron-boron magnetic powder can readily possess a high coerciveforce and provide a high magnetic flux density, although this magneticpowder comprises ultrafine granular or ellipsoidal particles.

Based on this finding, the present invention provides a magnetic tapecomprising a non-magnetic support, a magnetic layer containing magneticpowder which is formed on one side of the non-magnetic support, a primerlayer containing non-magnetic powder which is formed between thenon-magnetic support and the magnetic layer, and a backcoat layercontaining non-magnetic powder which is formed on the other side of thenon-magnetic support. This magnetic tape is provided as follows. Themagnetic powder used comprises plate, granular or ellipsoidal particleswith a particle diameter of 5 to 50 nm, preferably 5 to 30 nm, morepreferably 5 to 25 nm, and the thickness of the magnetic layercontaining this magentic powder is adjusted to 0.09 μm or less. At leastone of the primer layer and the backcoat layer contains non-magneticplate particles with a particle diameter (along the plate facedirection) of 10 to 100 nm.

In this regard, the particle diameter of magnetic particles and theparticle diameter of non-magnetic plate particles referred to in thecontext of the present specification (hereinafter referred to asnumber-average particle diameter, average particle diameter or averageparticle size) are determined by actually measuring the particle sizesof 500 particles shown on a photograph which was taken at amagnification of 250,000 with a transmission electron microscope (orTEM) and averaging the resultant 500 particle sizes.

The magnetic tape which uses such magnetic powder comprising plate,granular or ellipsoidal particles with a very fine particle size showsless magnetic interaction between each of the magnetic particles, andthus permits a rapid magnetic inversion and a narrower magneticinversion region. Therefore, it is found that this magnetic tape canobtain more excellent recording characteristics than a conventionalmagnetic tape which uses magnetic powder comprising needle particles.The magnetic recording medium according to the present inventionexhibits its effect particularly when the thickness of the magneticlayer is as thin as 0.09 μm or less. A recording medium having such athin magnetic layer shows less influence of demagnetization due to ademagnetizing field, and the resultant medium is found to show superiorrecording characteristics.

Examples of the magnetic powder having the above specified structure,that is, comprising ultrafine plate, granular or ellipsoidal particles,include barium ferrite magnetic powder and rare earth-iron-boronmagnetic powder, as mentioned above. Of these types of magnetic powder,the rare earth-iron-boron magnetic powder, which can readily possess ahigher coercive force and provide a higher magnetic flux density, willbe described in detail as a typical example thereof.

Also, the present inventors have intensively researched the formation ofa thinner primer layer. As a result, they have found that a primer layercan have an uniform thickness and a good surface smoothness bycontaining non-magnetic plate particles with a particle diameter (alongthe plate face) of 10 to 100 nm in the primer layer. In addition, it isalso found that the non-uniformity in the thickness of the magneticlayer is suppressed, since the disorder of the interface between theprimer layer and the magnetic layer is reduced.

To obtain the dimensional stability of the magnetic tape against changesin temperature and humidity, non-magnetic plate particles with aparticle diameter of 10 to 100 nm are contained in the primer layer,and/or non-magnetic plate particles with a particle diameter (along theplate face) of 10 to 100 nm are contained in the backcoat layer. Bydoing so, the superposition of the non-magnetic plate particlessuppresses the temperature/humidity expansion of a binder resin in thetape in-plane direction (the lengthwise direction and the widthwisedirection of the tape), and thus significantly improves the dimensionalstability in the tape widthwise direction against changes in temperatureand humidity, which is one of the problems of the present invention tobe solved.

Further, by containing the non-magnetic plate particles, variation inthe thickness of (the magnetic layer+the primer layer) and variation inthe thickness of the backcoat layer are lessened to thereby reduce thedeformation of a magnetic sheet wound up (the formation of raisedstripes, and dislocation of the edges of a wound tape), and thus, it isfound that edge weaves caused on the edges of tape when the magneticsheet is slit into tapes is reduced.

Magnetic tapes according to the second to fourth embodiments of thepresent invention can be suitably accomplished by employing theforegoing structures of the magnetic powder, the magnetic layer and thenon-magnetic plate particles, although employment of structures otherthan the above structures are not inhibited.

A magnetic tape according to the second embodiment comprises anon-magnetic support, a magnetic layer containing magnetic powder whichis formed on one side of the non-magnetic support, a primer layercontaining non-magnetic powder which is formed between the non-magneticsupport and the magnetic layer, and a backcoat layer containingnon-magnetic powder which is formed on the other side of thenon-magnetic support, which has a width of the recording track of 12 μmor less, and is used at a running speed of 4 m/sec. or higher,characterized in that the temperature expansion coefficient in the tapewidthwise direction is 0 to 8×10⁻⁶/° C., and the humidity expansioncoefficient, 0 to 10×10⁻⁶/% RH; and the amount of edge weave present oneither of the edges of the tape as the reference side for the running ofthe tape is less than 0.8 μm.

A magnetic tape according to the third embodiment comprises anon-magnetic support, a magnetic layer which containing magnetic powderwhich is formed on one side of the non-magnetic support, a primer layercontaining non-magnetic powder which is formed between the non-magneticsupport and the magnetic layer, and a backcoat layer containingnon-magnetic powder which is formed on the other side of thenon-magnetic support, characterized in that the thickness of themagnetic layer is 0.05 μm to 0.09 μm; and the rate of fluctuation inreproducing output in at least one of the tape lengthwise direction andthe tape widthwise direction is 8% or less when signals with awavelength of 2 μm are recorded on the magnetic tape with a magneticinduction type recording head having a recording track width of 76 μmand are reproduced with a magnetoresistance type reproducing head havinga track width of 38 μm (the thickness of a magnetoresistance typeelement: 0.05 μm).

Finally, a magnetic tape according to the fourth embodiment ischaracterized in that the thickness of the magnetic layer is 0.01 μm toless than 0.05 μm; and the rate of fluctuation in reproducing output inat least one of the tape lengthwise direction and the tape widthwisedirection is 10% or less when signals with a wavelength of 2 μm arerecorded on the magnetic tape with a magnetic induction type recordinghead having a recording track width of 76 μm and are reproduced with amagnetoresistance type reproducing head having a track width of 38 μm(the thickness of a magnetoresistance type element: 0.05 μm).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the arrangement of a whole of aslitting machine used for making magnetic tapes.

FIG. 2 is an enlarged partial sectional view of a suction roller in theslitting machine shown in FIG. 1.

FIG. 3 is a perspective view of a magnetic tape cartridge (a tape foruse in a computer) made in Examples of the present invention.

FIG. 4 shows a photograph of the particles of neodymium-iron-boron typemagnetic powder used in Example 1 of the present invention, which wastaken at a magnification of 250,000 with a transmission type electronicmicroscope.

DETAILED DESCRIPTION OF THE INVENTION

Magnetic powder comprising needle particles of an iron-cobalt alloy,which is conventionally used in magnetic recording media such as coatingtype high density magnetic tapes, has a theoretical limit in the valueof coercive force as the property (1), and also has a problem in theparticle size as the property (4), since it becomes very difficult touniformly disperse this magnetic powder if the powder is further reducedin particle size. However, the most serious problem is that it isessentially impossible to realize the properties (3) and (5) at the sametime, because the coercive force is induced by the shape magneticanisotropy due to the particle shape of needle, so that the an aspect isdecreased to about 5 as the smallest. If this ratio is below about 5,the uniaxial anisotropy of the powder lowers, resulting in a smallercoercive force.

The present inventors have synthesized a variety of magnetic powder withtaking into consideration the above essential properties in order toimprove the magnetic characteristics from the viewpoints different fromthe conventional magnetic powder based on the shape magnetic anisotropy.As a result of the examination of the magnetic anisotropy of theresultant magnetic powder, they have found that a rare earth-iron-borontype magnetic material, which comprises at least a rare earth, iron andboron as constitutive elements, has a large crystalline magneticanisotropy, and therefore is not needed to be formed into needleparticles. It is therefore known that this magnetic powder, even ifcomprising granular or ellipsoidal particles, can show a large coerciveforce in a single direction. The magnetic ellipsoidal particles referredto in the present invention mean that the ratio of the major axis to theminor axis of an ellipsoidal particle is 2 or less, and this shape ofthe particles is essentially different from the particle shapes of theconventional magnetic powder for use in magnetic recording media.

The rare earth-iron-boron magnetic powder is generally known as a highperformance magnetic material comprising particles in the order ofsubmicron, prepared by a powder metallurgy technique. For example, aneodymium-iron-boron magnetic material for use in a permanent magnet hasa composition represented by Nd₂Fe₁₄B, and has a very large coerciveforce of 800 kA/m or more. However, the coercive force of a magneticrecording medium is determined by the relationship with a recordinghead, and it is generally said that signals can be magnetically recordedon a magnetic recording medium having a coercive force which is up toabout one sixth of the saturation magnetic flux density of a magnetichead. Therefore, this magnetic material can not be used as magneticpowder for a magnetic recording medium, because of its excessivecoercive force which makes it impossible for a magnetic head to deleterecorded signals.

The present inventors have discovered that, based on the foregoingproperties, it is effective to decrease the amount of a rare earthelement added relative to iron, as compared with that of the rare earthelement for use in the above permanent magnet, and to increase theamount of boron relative to iron, in order to achieve a coercive forcesuitable for a magnetic recording medium. While, as the rareearth-iron-boron type magnetic material, a compound represented byNd₂Fe₁₄B as above is well known as showing a particularly high coerciveforce, the present inventors also have discovered that a coercive forcesufficient for a magnetic recording medium can be obtained by using, asa rare earth element, samarium (Sm), terbium (Tb) or yttrium (Y) inplace of Nd, in the above composition. That is, the present inventorshave firstly manifested that rare earth elements other than neodymiumwhich has attracted public attentions so far, can be used for magneticrecording media. In other words, the present inventors have paid keenattentions on the rare earth-iron-boron type magnetic materials whichhave been used for permanent magnets, as materials for use in magneticrecording media having a coercive force range lower than those ofpermanent magnets, and firstly have succeeded in putting them intopractical use. Thus, they have successfully developed quite a novelfield of materials.

The present inventors have advanced their researches on the rareearth-iron-boron type magnetic materials based on the above findings. Asa result, they have discovered that the following rare earth-iron-boronmagnetic powder can show a higher coercive force to an extent thatrecorded signals can be deleted by a magnetic head, and that it canimpart excellent electromagnetic conversing properties to a thin layercoating type magnetic recording medium. That is, the content of a rareearth element of a rare earth-iron-boron type magnetic material isdecreased as compared with the content thereof in the composition whichis known as a material for a permanent magnet, and further, rareearth-iron-boron magnetic powder is prepared as follows. Each magneticparticle of magnetic powder comprises a core portion of an iron metal oran iron alloy and an outer layer portion of a rare earth-iron-boroncompound, and has a particle diameter (an average particle size) of 5 to50 nm, preferably 5 to 30 nm, more preferably 5 to 25 nm, and a shape ofa grain or an ellipsoid. As the rare earth in the magnetic powder ofthis type, there is used at least one element selected from yttrium,ytterbium, cesium, praseodymium, samarium, lanthanum, europium,neodymium, terbium, and the like, and the use of neodymium, samarium,terbium or yttrium is effective for the magnetic powder to achieve ahigh coercive force.

It becomes possible for a thin layer coating type magnetic recordingmedium to show high coercive force and high saturation magnetization atthe same time by using such a specified rare earth-iron-boron magneticpowder in the coating type magnetic recording medium. That is, since theabove magnetic powder has a largely reduced content of a rare earthelement, or since each magnetic particle of the above magnetic powderhas a core portion of an iron metal or an iron alloy as a maincomponent, high saturation magnetization peculiar to the iron metal orthe iron alloy can be obtained. Particularly when the core portion ofthe magnetic particle is composed of an iron metal or an iron alloy, andspecifically when the core portion is composed of an iron-cobalt alloy,the highest saturation magnetization can be obtained. The use of theiron metal or the iron alloy alone lowers the coercive force of themagnetic powder because it has no shape anisotropy. However, theaddition of small amounts of a rare earth and boron thereto markedlyincreases the coercive force. In another case where each particle ofmagnetic powder has a core portion of an iron metal or an iron alloy andan outer layer portion of a rare earth-iron-boron compound whichencloses the core portion, the magnetic powder as a whole can show ahigh coercive force since this compound has a high coercive force. Inthis case, the compound itself shows a relatively low saturationmagnetization, while the iron metal or the iron alloy maintains highsaturation magnetization. As a result, the magnetic powder can achieveboth of high saturation magnetization and high coercive force at thesame time.

As described above, the above specified rare earth-iron-boron magneticpowder suitably used in the present invention exhibits excellentmagnetic characteristics. The reason therefor is considered that themagnetic anisotropy of the iron metal or the iron alloy is integratedwith the magnetic anisotropy of the rare earth-iron-boron compoundthrough their magnetic interaction, so that the magnetic powder behavesas if it were a single magnetic body, in spite of its particle structurehaving the core portion and the outer layer portion. The phenomenon ofthe integration of different kinds of magnetic anisotropy inside theparticle because of their magnetic interaction is an innovationaldiscovery which has not been anticipated from the common knowledge, andwhich has been firstly discovered by the present inventors.

The present inventors have made researches on the particle size of theabove rare earth-iron-boron magnetic powder. As a result, they havefound that a magnetic layer can achieve excellent magneticcharacteristics on condition that the particle diameter (the averageparticle size) of the magnetic powder is 5 to 50 nm, preferably 5 to 30nm, more preferably 5 to 25 nm. The conventional magnetic powder of theneedle particle type is required to have an average particle size ofabout 100 nm as the smallest in order to maintain a high coercive force.On the other hand, the coercive force of the above magnetic powdersuitably used in the present invention originates mainly from theanisotropy of the crystals, and therefore, the magnetic powder cancomprise very fine particles with an average particle size of up to 5nm, and each of such very fine particles can exhibit excellent magneticcharacteristics. The average particle size is more preferably 8 nm ormore, particularly 10 nm or more.

If the particle diameter (or the average particle size) of the abovemagnetic powder is too large, the amount of the magnetic powder to becontained in the magnetic layer decreases, and also, the surfacesmoothness of the magnetic layer is impaired, if the layer is formedwith a thin thickness. Furthermore, particle noises due to the size ofthe particles in the resultant magnetic recording medium become larger.Therefore, the average particle size of the magnetic powder should be 50nm or less, preferably 30 nm or less, more preferably 25 nm or less. Themagnetic powder with a particle size as selected above can have a veryhigh content of the powder and thus can achieve excellent saturationmagnetic flux density.

The iron metal or the iron alloy in the above rare earth-iron-boronmagnetic powder suitably used in the present invention contributes tohigh saturation magnetization. In case of the use of the iron alloy, atransition metal such as Mn, Zn, Ni, Cu, Co or the like is used as analloying metal. Among these transition metals, Co and Ni are preferable,and Co is particularly preferable because it contributes the most to theimprovement of saturation magnetization. The amount of a transitionmetal as above is preferably 5 to 50 atomic %, more preferably 10 to 30atomic % based on iron.

The amount of the rare earth in the rare earth-iron-boron compound is0.2 to 20 atomic %, preferably 0.3 to 15 atomic %, more preferably 0.5to 10 atomic %, based on the amount of the iron. The amount of the boronin the whole of the magnetic powder is 0.5 to 30 atomic %, preferably 1to 25 atomic %, more preferably 2 to 20 atomic % based on the amount ofthe iron. These atomic percentages are determined by fluorescent X-rayanalyses. The compounding of the above-specified amounts of the rareearth and the boron integrally enhances the bond in the particle becauseof the magnetic interaction of the different kinds of magneticanisotropy. As a result, a coercive force of 80 to 400 kA/m which isoptimal for magnetic powder for use in a high performance magneticrecording medium can be obtained.

Next, the particle shape of the above rare earth-iron-boron magneticpowder will be described from the viewpoint of the dispersibility of apaint for magnetic layer and the properties for forming a thin magneticlayer.

In the conventional magnetic powder of the needle particle type, theparticle size thereof is reduced so as to improve the recordingcharacteristics such as reduction of noises. The specific surface areaof the magnetic powder consequently increases, so that the interactionbetween the magnetic powder and a binder resin becomes larger, whichmakes it difficult to obtain a homogenous dispersion when the magneticpowder is dispersed in the binder resin. In addition, when thedispersion is diluted with a large amount of an organic solvent so as toform a thin coating layer, the magnetic powder tends to agglomeate, sothat the orientation and the surface smoothness of the resultant coatinglayer degrade. From this point of view, there is a limit in the particlesize of magnetic powder usable in a coating type magnetic recordingmedium.

By contrast, the rare earth-iron-boron magnetic powder suitably used inthe present invention has a particle shape of a grain or an ellipsoid,and therefore, it is possible to form particles in the shape ofsubstantial sphere which minimizes the specific surface area of themagnetic powder. Therefore, the interaction between the magnetic powderand a binder resin is smaller, as compared with the conventionalmagnetic powder, which leads to an improved flowability of a paint formagnetic layer. Accordingly, the dispersion of the magnetic powder, evenif having some agglomeration, becomes easy. Thus, this magnetic powdercan be used to prepare a paint for magnetic layer particularly suitablefor forming a magnetic layer by applying with a thin thickness. As aresult, magnetic powder with a particle size of so small as about 5 nmas mentioned above is sufficient for practical use in a magneticrecording medium.

As mentioned above, to avoid the influence of a decrease in output dueto demagnetization in the course of recording and reproducing, which isthe essential problem of the lengthwise recording, it is effective todecrease the thickness of the magnetic layer. However, there is a limitin the thickness of the magnetic layer, insofar as magnetic powder ofneedle particle type with a particle size of about 100 nm is used. Thisis described below. The needle particles of magnetic powder aregenerally arrayed so that the needle-pointed direction can be inparallel to the in-plane direction of a recording medium due to themagnetic orientation. However, this magnetic orientation has adistribution, so that some of the particles are so distributed thattheir needle-pointed direction can be perpendicular to the surface ofthe medium. Such needle particles project from the surface of themagnetic layer to impair the surface smoothness of the medium and tothereby markedly increase the noises. This problem becomes more and moreserious, as the thickness of the magnetic layer becomes more and morethinner. Therefore, in the state of the art, it is difficult to form acoating magnetic layer having a thickness of about 0.09 μm or less andalso a smooth surface, insofar as the needle particle type magneticpowder is used.

In case where a primer layer is formed between a non-magnetic supportand a magnetic layer so as to form the magnetic layer with a thinnerthickness, and where the simultaneous superposing type coating method inwhich a paint for magnetic layer containing the needle particle typemagnetic powder is applied on the primer layer in a wet state isemployed, the magnetic powder is drawn into the primer layer, so thatthe needle magnetic particles of the magnetic powder tend to projectinto the primer layer at the interface on the magnetic layer to therebyfurther disturb the orientation of the magnetic layer. Therefore, adesired squareness ratio can not be obtained, and also, the smoothnessof the surface of the magnetic layer tends to degrade. It is consideredthat this problem may constitute one of factors to make it hard for athin magnetic layer containing the needle particle type magnetic powderto achieve a high density.

By contrast, the rare earth-iron-boron magnetic powder suitably used inthe present invention has not only a smaller particle size but also aparticle shape of a grain or an ellipsoid which can take a shape ofsubstantial sphere. Therefore, the magnetic particles of this magneticpowder do not project from the surface of the magnetic layer, norproject into the primer layer, like the needle particle type magneticpowder. Thus, the surface smoothness of the resultant magnetic layer isvery good. In the meantime, the output decreases as the thickness of themagnetic layer is reduced, because the magnetic flux from the magneticlayer lessens. However, the magnetic powder used in the presentinvention has a great advantage to overcome this problem, because ofhaving a particle shape of a grain or an ellipsoid which can take ashape of substantial sphere. Thus, the magnetic powder used in thepresent invention can be packed in the magnetic layer at a highercontent as compared with the needle particle type magnetic powder. As aresult, the magnetic layer of the present invention can obtain a highermagnetic flux density.

Next, the saturation magnetization of the magnetic powder used in thepresent invention will be described. Magnetic powder of a metal or analloy generally tends to have a larger specific surface area, as theparticle size thereof becomes smaller, so that the ratio of the oxidizedsurface of the layer which does not contribute to saturationmagnetization becomes larger, and that the magnetic portion whichcontributes to the saturation magnetization becomes smaller. In otherwords, as the particle size of the magnetic powder becomes smaller,saturation magnetization becomes smaller. This tendency is especiallyremarkable in the needle particle type magnetic powder, in which thesaturation magnetization abruptly decreases when the major axis isaround 100 nm. Such a decrease in saturation magnetization is one of thefactors to determine the limit of the usable particle size.

By contrast, the rare earth-iron-boron magnetic powder used in thepresent invention has a particle shape of a grain or an ellipsoid, thespecific surface area of this magnetic powder becomes minimum, whencompared with other magnetic powder based on the same volume. Therefore,this magnetic powder, in spite of comprising very fine particles, canmaintain high saturation magnetization.

The wording of the particle shape of “a grain or an ellipsoid” of therare earth-iron-boron magnetic powder suitably used in the presentinvention means that the shapes of the particles of the magnetic powderinclude all the shapes from a substantially granular shape to anellipsoidal shape (including medium shapes between the granular shapeand the ellipsoidal shape), and the particle shape of the presentinvention may be any one selected therefrom. In other words, thiswording is used to exclude “the needle particle type” conventionalmagnetic powder. Among the above particle shapes, the spherical orellipsoidal particles are preferable, because the specific surface areaof the magnetic powder becomes the smallest. These particle shapes canbe observed with a transmission type electronic microscope, as well asthe particle size.

As described above, the rare earth-iron-boron magnetic powder isessentially suitable for obtaining a thin magnetic layer, in all theterms of saturation magnetization, coercive force, particle size, andparticle shape. When this magnetic powder is used to make a magneticrecording medium having a magnetic layer with an average thickness of0.09 μm or less (particularly 0.06 μm or less), especially excellentrecording/reproducing characteristics can be obtained. It is preferableto use magnetic powder having a saturation magnetization of 80 to 200A.m²/kg (80 to 200 emu/g), in order to improve the characteristics ofthe high recording density region of a magnetic recording mediumcomprising a magnetic layer with an average thickness of 0.09 μm orless.

In the context of the present specification, the coercive force and thesaturation magnetization of magnetic powder are values determined asfollows: the coercive force and the saturation magnetization of areference sample are measured with a sample-vibration type magnetometerat 25° C. under application of a magnetic field of 1,273.3 kA/m (16kOe), and the measured values are corrected for an intended magneticpowder.

For example, the rare earth-iron-boron magnetic powder suitably used inthe present invention is prepared by the following method. First, anaqueous solution which contains ions of a rare earth such as neodymium,samarium or the like, and iron ions, and if necessary, ions of atransition metal such as Mn, Zn, Ni, Cu, Co or the like is mixed with anaqueous alkaline solution to form a co-precipitate of the rare earth,the iron and the above transition metal. As the raw materials of therare earth ions, the iron ions and the transition metal ions, ironsulfate, iron nitrate or the like is used. Next, a boron compound isadded to the co-precipitate, and the mixture is heated at a temperatureof 60 to 400° C. to form a boron-containing oxide of the rare earth andthe iron (and optionally the above transition metal).

The above boron compound acts as a flux for growing crystals to anintended particle size while protecting the particles from excessivebaking, as well as a source for supplying boron. As such a boroncompound, H₃BO₃ or the like is preferably used, although not limitedthereto. The boron compound in a solid state may be mixed with theco-precipitate. However, in order to obtain magnetic powder having goodphysical properties, boron is dissolved and mixed in a suspension of theco-precipitate, and the mixture is dried to remove water and treated byheating.

Next, the heat-treated product is washed with water to remove excessiveboron, and dried. This product is reduced by heating at a temperature of400 to 800° C. under a hydrogen atmosphere or the like for reduction toobtain a rare earth-iron-boron magnetic powder. Other elements may becontained in this magnetic powder to improve the anti-corrosionproperty, etc. Also, in this case, it is desirable that the amounts ofthe rare earth and the boron in a whole of the magnetic powder are 0.2to 20 atomic % and 0.5 to 30 atomic %, based on the amount of the iron,respectively.

The magnetic powder of the present invention may be prepared by anothermethod. First, an aqueous solution which contains iron ions, and ifnecessary, ions of a transition metal such as Mn, Zn, Ni, Cu, Co or thelike is mixed with an aqueous alkaline solution to form a co-precipitateof the iron and optionally the above transition metal. As the rawmaterials of the iron ions and the transition metal ions, iron sulfate,iron nitrate or the like is used. Next, a rare earth element such asneodymium, samarium or the like, and a boron compound are added to theco-precipitate, and the mixture is heated at a temperature of 60 to 400°C. to form a boron-containing oxide of the rare earth and the iron (andoptionally the above transition metal). Then, the excessive boron isremoved from the heat-treated product, and the product is reduced byheating under a hydrogen atmosphere as in the foregoing method to obtainthe rare earth-iron-boron magnetic powder. This method is suitable forobtaining a rare earth-iron-boron magnetic powder in which each magneticparticle comprises a core portion of an iron metal or an iron alloy withthe above transition metal as a main component, and an outer layer of arare earth-iron-boron compound as a main component. Also in this method,other elements may be contained in this magnetic powder to improve theanti-corrosion property, etc. Also, in this case, it is desirable thatthe amounts of the rare earth and the boron in a whole of the magneticpowder are 0.2 to 20 atomic % and 0.5 to 30 atomic %, based on theamount of the iron, respectively.

In order to pack and disperse the ultrafine magnetic particles with aparticle size (an average particle diameter) of 50 nm or less in acoating layer at a high density, it is preferable to prepare a paint bythe following steps. Prior to the kneading step, the granular particlesof magnetic powder are cracked with a cracking machine, and the crackedparticles are mixed with an organic acid such as phosphoric acid, and abinder resin in a mixer, so as to treat the surfaces of the particles ofthe magnetic powder. In the kneading step, the treated mixture iskneaded in a continuous type twin screw kneader so that the solidcontent can be 80 to 85 wt. %, and that the ratio of the binder resin tothe magnetic powder can be 17 to 30 wt. %. In the next step after thekneading step, the continuous twin screw kneader or other dilutingmachine is used to knead and dilute the knead-mixture with a binderresin solution and/or a solvent, at least one time, and the resultantpaint is dispersed with a very fine media rotation type dispersingmachine such as a sand mill.

Hexagonal barium ferrite powder may be contained in the magnetic layer.The coercive force of the hexagonal barium ferrite powder added ispreferably 120 to 320 kA/mm, and the amount of saturation magnetizationis preferably 50 to 70 A.m²/kg (50 to 70 emu/g). The magneticcharacteristics of the magnetic layer and the ferromagnetic powder aremeasured in an external magnetic field of 1,273.3 kA/m (16 kOe) with asample vibration type fluxmeter.

It is preferable that the hexagonal barium ferrite powder has a particlediameter (along the plate face direction) of 5 to 50 nm, more preferably5 to 30 nm, further preferably 5 to 25 nm. If the particle diameter isless than 5 nm, the agglomerating force of the magnetic powderincreases, so that the dispersion of the magnetic powder in a paintbecomes difficult. On the other hand, if it exceeds 50 nm, particlenoises based on the size of the particles become larger. The plate ratio(the particle diameter/the thickness of the plate) is preferably 2 to10, more preferably 2 to 5, further preferably 2 to 4. In this regard,the particle diameter is found by actually measuring the sizes of 500particles on a photograph which is taken with a transmission electronmicroscope (TEM), and averaging the 500 particle sizes. The BET specificsurface area of the hexagonal barium ferrite powder is preferably 1 to100 m²/g.

Examples of the non-magnetic particles used in the primer layer includetitanium oxide, iron oxide, aluminum oxide, etc., and iron oxide aloneor a mixture of iron oxide and aluminum oxide is used. In general,non-magnetic iron oxide having a major axis of 0.05 to 0.2 μm and aminor axis of 5 to 200 nm is mainly used, and if necessary, carbon blackhaving a particle diameter of 0.01 to 0.1 μm, and aluminum oxide havinga particle diameter of 0.1 to 0.5 μm are auxiliarily contained in theprimer layer. The non-magnetic particles and the carbon black have notso sharp particle distributions, and this defect is not so serious whenthe thickness of the primer layer is 1.0 μm or more. However, when thethickness of the primer layer is 1.0 μm or less, the particles on thelarger particle diameter side of the particle distribution influence thesurface roughness of the primer layer. For this reason, it is difficultto form a thin primer layer with a thickness of 1.0 μm or less.

To overcome this problem, the present inventors have made intensiveresearches for obtaining non-magnetic plate particles which areultrafine particles having a small particle distribution and suitablefor a primer layer, such as aluminum oxide particles or the like. As aresult, they have succeeded in obtaining non-magnetic plate particleswith a particle diameter (a number-average particle diameter) of 10 to100 nm. The particle diameter of the non-magnetic plate particles isdetermined by actually measuring the particle sizes (the maximumparticle diameter along the plate face direction) of 500 particles on aphotograph which was taken at a magnification of 250,000 with atransmission electron microscope (TEM), and averaging them fornumber-average value.

The non-magnetic plate particles such as aluminum oxide plate particlesor the like with a particle size of 10 to 100 nm used in the presentinvention have major two features. One is that, because of beingultrafine plate particles, variation in the thickness of a coating layerwith a thickness as thin as 1.0 μm or less is small, so that thesmoothness of the interface between such a primer layer and a magneticlayer does not degrade. The other is that a coating layer in which theplate particles are superposed on one another is formed, so that thecoating layer is reinforced in the plane direction and is concurrentlyimproved in dimensional stability against changes in temperature andhumidity.

For example, aluminum oxide particles are produced by any of variousknown methods. In general, aluminum oxide formed by baking is pulverizedin a ball mill or the like to obtain fine particles thereof. Thealuminum oxide particles provided by this method, however, have a wideparticle size distribution, and the particle size is in the order ofsubmicron as the smallest because of the mechanical pulverization, sothat it is hard to obtain further fine particles of aluminum oxide.

A precipitate of aluminum hydroxide is formed by neutralization, andthis aluminum hydroxide is heated in an air to obtain aluminum oxideparticles. This method makes it possible to obtain aluminum oxideparticles with a small particle diameter. However, the shapes of theresultant particles are irregular, which makes it impossible to impart,to a layer containing the particles, surface smoothness, reinforcement,dimensional stability against changes in temperature and humidity, etc.In addition, there arises a tendency to form secondary particles due tothe inter-particle agglomeration. Therefore, a large energy and verylong time are required to obtain an uniform dispersion.

For example, JP-A-7-315833 describes that plate-shaped alumina formed bybaking is pulverized with a non-metallic medium for long time, so as todestruct the agglomeration of the particles. This method has a limit inthe size of the resultant very fine particles, since the particles areprovided by pulverization, and consequently, the resultant particleshave a wide particle size distribution.

On the other hand, the method of forming a plate-shaped alumina througha hydrothermal synthesis has long been known. For example, JP-B-37-7750and JP-B-39-13465 describe that the particle diameters of plate-shapedalumina obtained are in the order of several microns to several hundredmicrons, which has a problem in view of formation of very fineparticles.

Otherwise, aluminum hydroxide particles, the sizes of which have beenpreviously regulated in the order of submicron, are subjected to ahydrothermal treatment at a high temperature of 350° C. or more in wateror an aqueous alkaline solution to obtain aluminum oxide plate particlesin the order of submicron (for example, JP-A-5-17132 and JP-A-6-316413).This method makes use of a hydrothermal reaction which makes it easy toform plate-shaped aluminum excellent in crystallinity, so as to causecrystal modification for transforming aluminum hydroxide into aluminumoxide. Therefore, this reaction is carried out at a high temperature,and thus requires a special reaction vessel capable of withstandingunder a high pressure. This method is considered to be suitable forproducing aluminum oxide particles with a large particle diameter in theorder of submicron because of making use of a hydrothermal reactionunder a high temperature atmosphere, but unsuitable for producing veryfine aluminum oxide particles with a small particle diameter of 100 nmor less.

As described above, despite the many demands for very fine aluminumoxide particles with a good crystallinity and a particle diameter of 100nm or less and showing a sharp particle diameter distribution, aluminumoxide particles capable of satisfying the above requirements have notyet been developed.

The present inventors have newly developed very fine non-magnetic plateparticles such as plate-shaped aluminum oxide particles, which satisfythe above requirements, and found out that the use of this non-magneticplate particles for a primer layer makes it possible to reduce avariation in the thickness of a thin coating layer, and to improve thesmoothness of the interface between a primer layer and a magnetic layer,the strength of a coating layer in the plane face direction, dimensionalstability against changes in temperature and humidity, etc. In addition,similar effects (a decrease in variation in thickness, and improvementon surface smoothness, strength of a coating layer in the plane facedirection, dimensional stability against changes in temperature andhumidity, etc.) are also observed in a magnetic layer and a backcoatlayer. Accordingly, the addition of such non-magnetic plate particles toa magnetic layer and a backcoat layer is effective to provide a magnetictape which has very small variation in the thickness, a smooth surface,an improved strength, and high dimensional stability against changes intemperature and humidity, even though the magnetic tape has a magneticlayer with a thickness of 0.09 μm or less, preferably 0.06 μm or less,as in the present invention.

The variation in the thickness of a magnetic layer is measured by takinga photograph of the sliced section of a magnetic tape at a magnificationof 10,000 to 100,000 with a transmission electron microscope (TEM) andmeasuring a plurality of points on the sliced section to determine thevariation in the thickness of the magnetic layer, as disclosed inJP-A-2001-134919 and JP-A-2001-256633. This method, however, has thefollowing problems. When the magnetic tape is sliced, the coating layerdislocates; it is difficult to obtain a correct variation in thethickness of the magnetic layer due to the presence of disturbedportions on the interface between a primer layer and the magnetic layer;and the points to be measured are limited to a part of the tape, in caseof the measurement with an electronic microscope.

The present inventors have examined these problems and made thefollowing trial in order to solve the problems. Signals with wavelengthslong enough to the thickness of a magnetic coating layer are recorded ona magnetic tape to make magnetic record in the entire thickness of themagnetic layer, and fluctuations in the signal outputs are read tothereby take the thickness data as fluctuations in outputs. The abovesignals with wavelengths long enough to the thickness of the magneticcoating layer (for example, 10 times longer than the thickness of thelayer) are saturation-recorded on the magnetic coating layer, andtherefore, the signal outputs are proportional to the thickness of thecoating layer. As a result of comparison between this measuring methodand the conventional method of measuring the section of a tape on aphotograph, a correlation is found between the fluctuation in the signaloutputs and the variation in the thickness of the magnetic layer.Further, the data of fluctuation in signal outputs obtained by readingthe fluctuation in the signal outputs from 50 m length of a magnetictape at relatively narrow pitches (2.54 mm pitches along the tapelengthwise direction) also have a correlation with data obtained byreading them from a whole length of the magnetic tape in a tapecartridge at relatively wide pitches (25.4 mm pitches along the tapelengthwise direction). Then, fluctuation in signal outputs are read from50 m length of the magnetic tape at 2.54 mm pitches along the tapelengthwise direction, and the resultant values are used as indexes forthe variation in the thickness of the magnetic layer. In this measuringmethod, the position of a head is shifted in the widthwise direction tomake similar measurement in the same manner. By doing so, data offluctuation in outputs from different widthwise positions (variation inthickness) can be obtained.

Here, the newly developed method for producing non-magnetic plateparticles, as mentioned above, will be described by making reference toaluminum oxide particles as an example.

To obtain aluminum oxide particles suitable for a primer layer, in thefirst step, an aqueous solution of aluminum salt is added to an aqueousalkaline solution containing oxy alkali amine, and the resultanthydroxide or hydrate of aluminum is subjected to a hydrothermaltreatment by heating it at a temperature of 110 to 300° C. in thepresence of water, so as to regulate the resultant particles to anintended shape and an intended particle diameter.

The problem of this step rests in the peculiar property of the hydroxideor hydrate of aluminum that the hydroxide or hydrate of aluminum can bedissolved both in an alkaline solution and an acidic solution, and formsits precipitate only at or around neutral pH. However, to obtainparticles of a hydroxide or a hydrate of aluminum having an intendedshape and an intended particle diameter through a hydrothermal reaction,it is needed to use an alkaline solution. The present inventors haveintensively studied in order to overcome the problem of the peculiarproperty of the hydroxide or hydrate of aluminum which has a trade-offrelationship. As a result, they have discovered that the intendedreaction can proceed only at or around pH 10.

Next, in the second step, the above hydroxide or hydrate of aluminum isheated in an air. By doing so, there can be obtained plate-shapedaluminum oxide particles with good crystallinity which show an uniformparticle diameter distribution and which is hardly sintered oragglomerated.

Thus, quite a novel conception for the production of aluminum oxideparticles which comprises separate steps is provided: that is, a stepfor regulating the shape and particle diameter of the particles iscarried out separately from a step for fully extracting the inherentphysical properties of a material. Based on this novel conception, thepresent inventors have succeeded in the development of plate-shapedaluminum oxide particles with a particle diameter (an average particlediameter) of 10 to 100 nm which any of the conventional processes hasnever achieved. The term “plate-shaped” referred to herein means a shapehaving a plate ratio (the maximal diameter/the thickness) of exceeding2. Preferably, this plate ratio is 100 or less, more preferably 3 to 50,further preferably 4 to 30, and most preferably 5 to 10. If the plateratio is 2 or less, some of the particles are raised from the surface ofa coating layer, in case of using a primer layer, so that the surfacesmoothness of the coating layer is impaired, while, if it exceeds 100,some of the particles are crushed during the preparation of a paint byusing the same.

This novel process comprising the separate steps as mentioned above canbe applied not only to aluminum oxide particles but also to theparticles of oxides or compounded oxides of rare earth elements such ascerium, elements such as zirconium, silicon, titanium, manganese, ironand the like, or their mixed crystals, which have a particle diameter(number-average particle diameter) of 5 to 100 nm.

The product of the residual magnetic flux density and the thickness of amagnetic layer in the tape lengthwise direction is preferably 0.0018 to0.05 μTm, more preferably 0.0036 to 0.05 μTm, further preferably 0.004to 0.05 μTm. If this product is less than 0.0018 μTm, the reproducingoutput with a MR head is poor, while, if it exceeds 0.05 μTm, thereproducing output tends to skew. The use of a magnetic recording mediumhaving such a magnetic layer advantageously makes it possible to shortenthe recording wavelength, to increase the reproducing output with a MRhead, to suppress the skew of the reproducing output and to increase theratio of output to noises.

It has already been described that the dimensional stability of amagnetic tape against changes in temperature and humidity can beimproved and the edge weave amount of the tape can be decreased bycontaining plate particles with a particle diameter (number-averageparticle diameter) of 10 to 100 nm in a primer layer and/or a backcoatlayer. The present inventors have further examined a slitting machine(100) shown in FIG. 1, which is used as a means for slitting a magneticsheet into a plurality of magnetic tapes with a predetermined width, andsucceeded in decreasing the edge weave amounts of the magnetic tapes.

They have investigated causes for forming, on a magnetic tape, an edgeweave with a short cycle (e.g., 50 mm or less) which is enough to causeoff-track at a tape-feeding rate of about 4 m/sec. As a result, theyhave discovered that the edge weave is caused in the magnetic tape by ashort cycled fluctuation in the tension of the tape induced by thefluttering of the magnetic sheet G which is being slit. Based on thisfinding, they have improved the components constructing the slittingmachine. Specifically, the slitting machine (100) shown in FIG. 1 isimproved in the tension cut roller (50) which is arranged in the webroute from a position for drawing out the magnetic sheet to the slitcutter group (61, 62) of the cutter-driving section (60); the timingbelt coupling (not shown) for transmitting power to the cutter-drivingsection (60); and reduction of the mechanical vibration of thecutter-driving section (60). In FIG. 1, numerals 90 and 91 refer toguides arranged along the feeding route for the magnetic sheet G.

As a result of the above improvements, the amount of edge weave with ashort cycle (cycle f=50 mm or less) formed on either of the edges of theslit magnetic tape (3) can be largely reduced. In the aboveimprovements, the most effective is the use of a mesh suction roller inwhich the suction holes (51) of the suction roller, which is the tensioncut roller (50) used for controlling the tension of the magnetic sheetis formed with a porous material, as shown in FIG. 2. The use of thismesh suction roller is effective to prevent dislocation in the tapewidthwise direction due to the edge weave with a short cycle. Thesuction roller section shown in FIG. 2 has suction holes (51) which arecommunicated with a suction source to suck the magnetic sheet, andtape-contacting portions (52) on the outer circumferential surface tocontact the magnetic sheet thereon. The suction holes (51) and thetape-contacting portions (52) are arranged alternately at regularintervals along the outer circumferential surface of the tension cutroller (50).

A cause for forming an edge weave with a cycle (e.g., 60 to 70 mm) whichtends to induce off-track at a tape-feeding rate of about 6 m/sec. wasinvestigated. As a result, it is found that the timing belt fortransmitting power to the cutter-driving section and the coupling havecaused such an edge weave. Thus, by using a flat belt in place of thetiming belt, and a rubber coupling in place of the metal coupling, anedge weave with a medium cycle could be largely reduced.

Further, a method for reducing an edge weave with a relatively longcycle has been investigated. As a result, it is known that the edgeweave amount could be extremely small by directly driving thecutter-driving section from a motor without using a power-transmittingdevice.

Further, researches have been made on a method for elongating the cycleof an edge weave to, for example, 100 mm or more, within which range,off-track is not caused even at a high tape-feeding rate of 8 m/sec. ormore. As a result, it is known that, by increasing the slitting speed,the cycle f can be elongated in accordance with the ratio of theslitting speed, which leads to less influence on the off-track, althoughthe edge weave amount is hardly changed.

Next, the components of a magnetic tape according to the presentinvention will be described in detail.

<Non-Magnetic Support>

The thickness of a non-magnetic support is generally 2 to 5 μm,preferably 2.5 to 4.5 μm, which may vary in accordance with an end use.When the thickness of the non-magnetic support is less than 2 μm, it isdifficult to form a film, and the strength of the resultant magnetictape tends to lower. When the thickness of the non-magnetic supportexceeds 5 μm, the total thickness of the magnetic tape increases so thatthe recording capacity per reel decreases.

The Young's modulus of the non-magnetic support in the lengthwisedirection is preferably at least 9.8 GPa (1,000 kg/mm²), more preferablyat least 10.8 GPa (1,100 kg/mm²). When the Young's modulus of thesupport is less than 9.8 GPa (1,000 kg/mm²), the travelling feeding ofthe magnetic tape may become unstable. In case of a helical scan typemagnetic tape, the ratio of the Young's modulus in the lengthwisedirection (MD) to the Young's modulus in the widthwise direction (TD) ispreferably 0.60 to 0.80, more preferably 0.65 to 0.75. When this ratiois less than 0.60 or when it exceeds 0.80, fluctuation in output fromthe region between the entrance to a track for a magnetic head and theexit from the track therefor (flatness) becomes larger. The flatnessbecomes minimum when the MD/TD ratio is at or around 0.70. Further, incase of a linear recording type magnetic tape, the ratio of the Young'smodulus in the lengthwise direction to the Young's modulus in thewidthwise direction is preferably 0.70 to 1.30. Examples of anon-magnetic support satisfying the above requirements are a biaxialoriented film of aromatic polyamide, aromatic polyimide, and the like.

<Primer Layer>

The thickness of a primer layer is preferably 0.2 to 1.0 μm, morepreferably 0.8 μm or less, particularly 0.5 μm or less. When thethickness of the primer layer is less than 0.2 μm, the effect ofreducing a variation in the thickness of a magnetic layer and the effectof improving the durability of the magnetic layer are poor. When thethickness of the primer layer exceeds 1.0 μm, the total thickness of amagnetic tape is too thick, so that the recording capacity per one reelof the magnetic tape decreases.

The primer layer may contain the above non-magnetic plate particles witha particle diameter of 10 to 100 nm so as to ensure the uniformity inthe thickness of the layer and the surface smoothness, and to controlthe stiffness and the dimensional stability of the tape.

The components of the non-magnetic particles are oxides or compoundedoxides of rare earth elements such as cerium, elements such aszirconium, silicon, titanium, manganese and iron, in addition toaluminum oxide. To improve the conductivity, plate-shaped ITO particles(indium tin oxide) are added. The plate-shaped ITO particles are addedin an amount of 15 to 75 wt. % based on the weight of all the inorganicpowder in the primer layer. Carbon such as plate-shaped graphiteparticles with a particle diameter of 10 to 100 nm may be used insteadof the plate-shaped ITO particles. If needed, carbon black may be added,and carbon black with a particle diameter of 10 to 100 nm is preferable.Further, conventional oxide particles of iron oxide, aluminum oxide andthe like may be added. In this case, it is preferable to use particlesas fine as possible (e.g., 10 to 100 nm). A binder resin used in theprimer layer may be the same one as used in the magnetic layer.

<Lubricant>

Preferably, the primer layer contains 0.5 to 5.0 wt. % of a higher fattyacid and 0.2 to 3.0 wt. % of a higher fatty acid ester based on thetotal weight of the powder components in the magnetic layer and theprimer layer, because the coefficient of friction of the magnetic tapeagainst a head can be decreased. When the amount of the higher fattyacid is less than 0.5 wt. %, the effect to decrease the coefficient offriction is insufficient. When the amount of the higher fatty acidexceeds 5.0 wt. %, the primer layer may be plasticized and thus thetoughness of the primer layer may be lost. When the amount of the higherfatty acid ester is less than 0.2 wt. %, the effect to decrease thecoefficient of friction is insufficient. When the amount of the higherfatty acid ester exceeds 3.0 wt. %, the amount of the higher fatty acidester which migrates to the magnetic layer becomes too large, so thatthe magnetic tape may stick to the head.

It is preferable to use a fatty acid having 10 or more carbon atoms.Such a fatty acid may be a linear or branched fatty acid, or an isomerthereof such as a cis form or trans form. However, a linear fatty acidis preferable because of its excellent luburicity. Examples of such afatty acid include lauric acid, myristic acid, stearic acid, palmiticacid, behenic acid, oleic acid, linoleic acid, etc., among whichmyristic acid, stearic acid and palmitic acid are preferable. The amountof the fatty acid to be added to the magnetic layer is not particularlylimited, since the fatty acid migrates between the primer layer and themagnetic layer. Thus, the sum of the fatty acids added to the magneticlayer and the primer layer is adjusted to the above-specified amount.When the fatty acid is added to the primer layer, the magnetic layer maynot always contain the fatty acid.

The coefficient of friction of the travelling magnetic tape can bedecreased, when the magnetic layer contains 0.5 to 3.0 wt. % of a fattyacid amide and 0.2 to 3.0 wt. % of a higher fatty acid ester, based onthe weight of the magnetic powder. When the amount of the fatty acidamide is less than 0.5 wt. %, the direct contact of the head and the themagnetic layer at their interface tends to occur, and thesintering-preventive effect is poor. When the amount of the fatty acidamide exceeds 3.0 wt. %, the fatty acid amide may bleed out and causes adefect such as dropout.

As the fatty acid amide, fatty acid amides each having at least 10carbon atoms such as the amides of palmitic acid, stearic acid and thelike can be used.

The addition of less than 0.2 wt. % of a higher fatty acid ester isinsufficient for decreasing the coefficient of friction, while theaddition of 3.0 wt. % or more of a higher fatty acid ester gives anadverse influence such as adhesion of the magnetic tape to the head orthe like. The intermigration of the lubricant between the magnetic layerand the primer layer is not always inhibited.

<Magnetic Layer>

The thickness of the magnetic layer is preferably from 0.01 to 0.09 μm,more preferably 0.06 μm or less, further preferably 0.04 μm or less

When the thickness of the magnetic layer is less than 0.01 μm, theresultant output is poor, and it is difficult to form a uniform magneticlayer. When the thickness of the magnetic layer exceeds 0.09 μm, theresolution to recorded signals with short wavelengths degrades.

The degree of variation in the thickness of the magnetic layer isevaluated as follows. Signals with a wavelength of 2 μm are recorded on50 m length of a magnetic tape with a magnetic induction type recordinghead having recording tracks with a width of 76 μm, and the recordedsignals are reproduced with a magnetoresistance type reproducing headhaving tracks with a width of 38 μm (the thickness of amagnetoresistance element: 0.05 μm). The reproducing outputs are read atconstant intervals, and the amount of fluctuation in output is measuredfor evaluation of the variation in the thickness of the magnetic layer.The amount of fluctuation in output in the tape lengthwise direction isdefined, for example, by the following:

-   -   (1) the average value of average fluctuation rate (%)=(an        absolute value of (output from each point−average output)/the        value of average output)×100″, and    -   (2) the standard deviation of output (%)=((standard deviation of        outputs from all the points)/the value of average output)×100.

The degree of fluctuation in output in the widthwise direction can besimilarly measured by shifting the position of a track for recording andreproducing a signal with wavelength of 2 μm in the widthwise direction.The average fluctuation rate of the output is 8% or less, preferably 6%or less in the lengthwise direction or the widthwise direction of thetape, on condition that the thickness of the magnetic layer is 0.05 to0.09 μm; and the average fluctuation rate of the output is 10% or less,preferably 8% or less in the lengthwise direction or the widthwisedirection of the tape, on condition that the thickness of the magneticlayer is 0.01 to less than 0.05 μm.

As a binder resin to be contained in the magnetic layer (or the primerlayer), the following can be used: a combination of a polyurethane resinwith at least one resin selected from the group consisting of a vinylchloride resin, a vinyl chloride-vinyl acetate copolymer, a vinylchloride-vinyl alcohol copolymer, a vinyl chloride-vinyl acetate-vinylalcohol copolymer, a vinyl chloride-vinyl acetate-maleic anhydridecopolymer, a vinyl chloride-hydroxyl group-containing alkyl acrylatecopolymer, and cellulose resins such as nitrocellulose. Among them, acombination of a vinyl chloride-hydroxyl group-containing alkyl acrylatecopolymer resin with a polyurethane resin is preferably used. Examplesof the polyurethane resin include polyesterpolyurethane,polyetherpolyurethane, polyetherpolyesterpolyurethane,polycarbonatepolyurethane, polyestrepolycarbonatepolyurethane, etc.

Preferably, a binder resin such as a urethane resin which is a polymerhaving, as a functional group, —COOH, —SO₃M, —OSO₃M, —P═O(OM)₃,—O—P═O(OM)₂ [wherein M is a hydrogen atom, an alkali metal base or anamine salt], —OH, —NR¹R², —N⁺R³R⁴R⁵ [wherein R¹, R², R³, R⁴ and R⁴ are,each independently the same or different, a hydrogen atom or ahydrocarbon group], or an epoxy group is used. The reason why such abinder resin is used is that the dispersibility of the magnetic powder,etc. is improved as mentioned above. When two or more resins are used incombination, it is preferable that the polarities of the functionalgroups of the resins are the same. In particular, the combination ofresins both having —SO₃M groups is preferable.

Each of these binder resins is used in an amount of 7 to 50 wt. parts,preferably from 10 to 35 wt. parts, based on 100 wt. parts of themagnetic powder. In particular, the best combination as the binder resinis 5 to 30 wt. parts of a vinyl chloride-based resin and 2 to 20 wt.parts of a polyurethane resin.

It is preferable to use the binder resin in combination with a thermallycurable crosslinking agent which bonds with the functional groups in thebinder to crosslink the same. Preferable examples of the crosslinkingagent include isocyanates such as tolylene diisocyanate, hexamethylenediisocyanate, and isophorone diisocyanate; and polyisocyanates such asreaction products of these isocyanates with compounds each having aplurality of hydroxyl groups such as trimethylolpropane, andcondensation products of these isocyanates. The crosslinking agent isused usually in an amount of 1 to 30 wt. parts, preferably 5 to 20 wt.parts, based on 100 wt. parts of the binder resin. When the magneticlayer is applied on the primer layer by the wet on wet method, some ofpolyisocyanate is diffused and fed from a paint for the primer layer.Therefore, the magnetic layer can be cross-linked to some degree, evenif polyisocyanate is not used in combination.

The magnetic layer may contain the above-mentioned non-magnetic plateparticles with a particle diameter (number-average particle diameter) of10 to 100 nm. If needed, the magnetic layer may contain a conventionalabrasive. Examples of such an abrasive include α-alumina, β-alumina,silicon carbonate, chrome oxide, cerium oxide, α-iron oxide, corundum,artificial diamond, silicon nitride, silicon carbonate, titaniumcarbide, titanium oxide, silicon dioxide, boron nitride, and the like.Each of these abrasives with Moh's hardness of 6 or more is used aloneor in combination. In case of a thin magnetic layer with a thickness of0.01 to 0.09 μm, the particle diameter (number-average particlediameter) of abrasive is preferably 10 to 150 nm. The amount of abrasiveto be added is preferably 5 to 20 wt. %, more preferably 8 to 18 wt. %based on the weight of the magnetic powder.

The magnetic layer may further contain plate-shaped ITO particles whichare prepared by the above-described method, plate-shaped carbon blackparticles, and conventional carbon black (CB) in order to improve theconductivity and the surface lubricity. Examples of such carbon blackinclude acetylene black, furnace black, thermal black and the like. Theparticle diameter (number-average particle diameter) thereof ispreferably 10 to 100 nm. If the particle diameter is 10 nm or less, itbecomes hard to disperse carbon black. If it is 100 nm or more, it isneeded to add a large amount of carbon black. In either case, thesurface of the magnetic layer becomes coarse, which leads to a decreasein output. The amount of carbon black to be added is preferably 0.2 to 5wt. %, more preferably 0.5 to 4 wt. %, based on the weight of themagnetic powder.

<Backcoat Layer>

To improve the tape-running performance, a backcoat layer may be formedon the other side of the above non-magnetic support composing themagnetic tape of the present invention (the side opposite to the side ofthe non-magnetic support on which the magnetic layer is formed). Thethickness of the backcoat layer is preferably from 0.2 to 0.8 μm. Whenthe thickness of the backcoat layer is less than 0.2 μm, the effect toimprove the tape-running performance is insufficient. When the thicknessof the backcoat layer exceeds 0.8 μm, the total thickness of themagnetic tape increases, so that the recording capacity per one reel ofthe tape decreases.

As carbon black (CB) to be contained in the backcoat layer, acetyleneblack, furnace black, thermal black or the like can be used. In general,carbon black with a small particle diameter and carbon black with alarge particle diameter are used in combination. The particle diameter(number-average particle diameter) of small particle diameter carbonblack is from 5 to 200 nm, preferably from 10 to 100 nm. When theparticle diameter of small particle diameter carbon black is less than 5nm, the dispersion thereof is difficult. When the particle diameter ofsmall particle diameter carbon black exceeds 200 nm, a large amount ofcarbon black should be added. In either case, the surface of thebackcoat layer becomes coarse and thus the surface roughness of thebackcoat layer may be transferred to the reverse side of the magneticlayer (embossing).

When the large particle diameter carbon black having a particle diameterof 300 to 400 nm is used in an amount of 5 to 15 wt. % based on theweight of the small particle diameter carbon black, the surface of thebackcoat is not roughened and the effect to improve the tape-runningperformance is increased. The total amount of the small particlediameter carbon black and the large particle diameter carbon black ispreferably from 60 to 98 wt. %, more preferably from 70 to 95 wt. %,based on the weight of the inorganic powder in the backcoat layer. Thecenter line average height Ra of the surface roughness of the backcoatlayer is preferably from 3 to 8 nm, more preferably from 4 to 7 nm. Thebackcoat layer is generally made non-magnetic, because, if the backcoatlayer has magnetism, there is a danger of disturbing the magneticsignals on the magnetic recording layer.

Further, the above non-magnetic plate particles having a particlediameter (number-average particle diameter) of 10 to 100 nm may be addedto the backcoat layer in order to improve the strength and thedimensional stability against changes in temperature and humidity. Thecomponents of the non-magnetic plate particle include not only aluminumoxide but also oxides or compounded oxides of rare earth elements suchas cerium, and elements such as zirconium, silicon, titanium, manganese,iron and the like. Further, the ITO plate particles (indium tin oxide)prepared by the above method, and plate carbon black particles may beadded to the backcoat layer in order to improve the conductivity of themagnetic tape. The plate ITO particles and the carbon black particlesare added to the backcoat layer in a total amount of 60 to 98 wt. %based on the weight of all the inorganic powder in the backcoat layer.The particle diameter (number-average particle diameter) of the carbonblack particles is preferably 10 to 100 nm. If needed, iron oxideparticles with a particle diameter of 0.1 to 0.6 μm may be added in anamount of 2 to 40 wt. %, preferably 5 to 30 wt. % based on the weight ofall the inorganic powder in the backcoat layer.

As a binder resin to be contained in the backcoat layer, the same resinsas the binder resins used in the magnetic layer and the primer layer canbe used. Among those, the combination of a cellulose resin with apolyurethane resin is preferably used so as to decrease the coefficientof friction and to improve the tape-running performance.

The amount of the binder resin in the backcoat layer is usually from 40to 150 wt. parts, preferably from 50 to 120 wt. parts, more preferablyfrom 60 to 110 wt. parts, still more preferably from 70 to 110 wt.parts, based on 100 wt. parts of the total of the carbon black and theinorganic non-magnetic powder in the backcoat layer. When the amount ofthe binder resin is less than 40 wt. parts, the strength of the backcoatlayer is insufficient. When the amount of the binder resin exceeds 150wt. parts, the coefficient of friction tends to increase. Preferably, 30to 70 wt. parts of a cellulose resin and 20 to 50 wt. parts of apolyurethane resin are used in combination. To cure the binder resin, acrosslinking agent such as a polyisocyanate compound is preferably used.

The crosslinking agent to be contained in the backcoat layer may be thesame as those used in the magnetic layer and the primer layer. Theamount of the crosslinking agent is usually from 10 to 50 wt. parts,preferably from 10 to 35 wt. parts, more preferably from 10 to 30 wt.parts, based on 100 wt. parts of the binder resin. When the amount ofthe crosslinking agent is less than 10 wt. parts, the film strength ofthe backcoat layer tends to decrease. When the amount of thecrosslinking agent exceeds 35 wt. parts, the coefficient of dynamicfriction of the backcoat layer against SUS increases.

<Organic Solvent>

Examples of organic solvents to be used in the paints for the magneticlayer, the primer layer and the backcoat layer include ketone solventssuch as methyl ethyl ketone, cyclohexanone, methylisobutylketone, etc.;ether solvents such as tetrahydrofuran, dioxane, etc.; and acetatesolvents such as ethyl acetate, butyl acetate, etc. Each of thesesolvents may be used alone or in combination, and such a solvent may befurther mixed with toluene for use.

EXAMPLES

The present invention will be explained in detail by way of thefollowing Examples, which do not limit the scope of the presentinvention in any way. In Examples and Comparative Examples, “parts” are“wt. parts”, and “average particle diameter” is “particle diameter(number-average particle diameter”, unless otherwise specified.

Example 1

<<Synthesis of Ultrafine Magnetic Particles>>

Iron nitrate (III) (0.074 mol) and neodymium nitrate (0.002 mol) weredissolved in water (600 cc). Separately from this aqueous solution ofnitrates, sodium hydroxide (0.222 mol) was dissolved in water (600 cc).To this aqueous sodium hydroxide solution, was added the aqueous nitratesolution, and the mixture was stirred for 5 minutes to form a hydroxide(co-precipitate) of iron and neodymium. The hydroxide was washed withwater and filtered to collect the hydroxide. To this hydroxide(containing water), were further added water (30 cc) and boric acid(H₃BO₃) (0.5 mol), and the mixture was heated at 60° C. in an aqueousboric acid solution, while the hydroxide of iron and neydymium was beingagain dispersed. The resultant dispersion was spread onto a vat anddried at 60° C. for 4 hours to evaporate off water. Thus, a homogenousmixture of the hydroxide of iron and neodymium and boric acid wasprepared.

This mixture was crushed and put into an alumina crucible. The mixturewas treated by heating at 200° C. in an air for 4 hours to obtain aneodymium-iron oxide to which boron was bonded. In this reaction, boricacid acts as a source for supplying boron and also as a flux for growingcrystals to an intended particle size while preventing the excessivesintering of the particles. The resultant product was washed with waterto remove excessive boron. Thus, particles of neodymium-iron oxide towhich boron was bonded were obtained. The oxide particles were reducedby heating at 450° C. in a hydrogen stream for 4 hours to obtainneodymium-iron-boron type magnetic powder. The resultant magnetic powderwas then cooled to room temperature in the stream of a hydrogen gas, andagain heated to 60° C. under an atmosphere of a mixed gas of nitrogenand oxygen, and treated for stabilization under the atmosphere of amixed gas of nitrogen and oxygen for 8 hours, and finally exposed to anair for use.

The resultant neodymium-iron-boron magnetic powder was subjected to afluorescent X-ray analysis, and it was found that the content ofneodymium to iron was 2.4 atomic %, and the content of boron to iron,9.1 atomic %. This magnetic powder was observed with a transmission typeelectronic microscope at a magnification of 250,000. As a result, thepowder comprised substantially spherical or ellipsoidal particles with aparticle diameter of 25 nm (hereinafter, the particle diameter isindicated by number-average particle diameter, average particle diameteror average particle size), as shown in the photograph of FIG. 4. Thesaturation magnetization of the magnetic powder under the application ofa magnetic field of 1,273.3 kA/m was found to be 132 A.m²/kg (132emu/g), and the coercive force thereof, 191.8 kA/m (2,410 Oe).

<<Synthesis of Plate Alumina Particles>>

Sodium hydroxide (0.075 mol) and 2-aminoethanol (10 ml) were dissolvedin water (80 ml) to prepare an aqueous alkaline solution. Separatelyfrom this alkaline solution, an aqueous aluminum chloride solution wasprepared by dissolving aluminum chloride (III) heptahydrate (0.0074 mol)in water (40 ml). To the aqueous alkaline solution, was added dropwisethe aqueous aluminum chloride solution to form a precipitate containingaluminum hydroxide. To the precipitate, was added dropwise hydrochloricacid so as to adjust the pH to 10.2. A suspension of the precipitate wasaged for 20 hours, and then washed with water in an amount about 1,000times larger than the amount of the precipitate.

Next, the supernatant was removed, and the pH of the residual suspensionof the precipitate was adjusted to 10.0 with an aqueous sodium hydroxidesolution. The suspension was charged in an autoclave and subjected to ahydrothermal treatment at 200° C. for 2 hours.

The resultant product was filtered and dried at 90° C. in an air. Thedried product was slightly crushed in a mortar and treated by heating at600° C. in an air for one hour. Thus, aluminum oxide particles wereobtained. After the heat treatment, the aluminum oxide particles werewashed with water, using an ultrasonic dispersing machine, so as toremove the non-reacted product and residues. Then, the particles werefiltered and dried.

The resultant aluminum oxide (or alumina) particles were subjected to anX-ray diffraction analysis. As a result, a spectrum corresponding toγ-alumina was observed. Further, the shapes of the particles wereobserved with a transmission electron microscope. As a result, they werefound to be square plate particles having a particle distribution of asnarrow as 30 to 50 nm (average particle diameter: 40 nm).

The resultant aluminum oxide particles were further treated by heatingat 1,250° C. in an air for one hour. The resultant aluminum oxideparticles were subjected to an X-ray diffraction analysis. As a result,a spectrum corresponding to α-alumina was observed. Further, the shapesof the particles were observed with a transmission electron microscope.As a result, they were found to be square plate particles having aparticle distribution of as narrow as 40 to 60 nm (average particlediameter: 50 nm).

<<Synthesis of Plate ITO Particles>>

Sodium hydroxide (0.75 mol) and 2-aminoethanol (100 ml) were dissolvedin water (800 ml) to prepare an aqueous alkaline solution. Separatelyfrom this alkaline solution, an aqueous solution of tin chloride andindium chloride was prepared by dissolving indium chloride (III)tetrahydrate (0.067 mol) and tin chloride (IV) pentahydrate (0.007 mol)in water (400 ml). To the former aqueous alkaline solution, was addeddropwise the latter aqueous solution of tin chloride and indium chlorideto form a precipitate of a hydroxide or a hydrate comprising tin andindium. The pH of the precipitate was 10.2. A suspension of theprecipitate was aged at room temperature for 20 hours, and then washedwith water until the pH of the precipitate reached 7.6.

Next, to the suspension of the precipitate, was added an aqueous sodiumhydroxide solution to adjust the pH to 10.8, and the suspension wascharged in an autoclave and subjected to a hydrothermal treatment at200° C. for 2 hours.

The resultant product was washed with water until its pH reached 7.8,and filtered and dried at 90° C. in an air. The dried product wasslightly crushed in a mortar and treated by heating at 600° C. in an airfor one hour. Thus, particles of tin-containing indium oxide (ITOparticles) were obtained. After the heat treatment, the tin-containingindium oxide particles were washed with water, using an ultrasonicdispersing machine, so as to remove the non-reacted product andresidues. Then, the particles were filtered and dried.

The shapes of the resultant tin-containing indium oxide particles wereobserved with a transmission electron microscope. As a result, they werefound to be disc or square plate particles having a particledistribution of as narrow as 30 to 50 nm (average particle diameter: 40nm).

The tin-containing indium oxide particles were subjected to an X-raydiffraction analysis. As a result, the X-ray diffraction spectrumindicated that the particles were composed of a single structuresubstance which was the tin-containing indium oxide in which indium wassubstituted by tin.

<<Synthesis of Plate Non-Magnetic Iron Oxide Particles>>

Sodium hydroxide (0.75 mol) and 2-aminoethanol (100 ml) were dissolvedin water (800 ml) to prepare an aqueous alkaline solution. Separatelyfrom this alkaline solution, an aqueous ferric chloride solution wasprepared by dissolving ferric (III) chloride hexahydrate (0.074 mol) inwater (400 ml). To the former aqueous alkaline solution, was addeddropwise the latter aqueous ferric chloride solution to form aprecipitate containing ferric hydroxide which had a pH of 11.3. Asuspension of the precipitate was aged for 20 hours and washed withwater until its pH reached 7.5.

Next, the supernatant was removed, and the suspension of the precipitatewas charged in an autoclave and subjected to a hydrothermal treatment at150° C. for 2 hours.

The resultant product was filtered and dried at 90° C. in an air. Thedried product was slightly crushed in a mortar and treated by heating at600° C. in an air for one hour. Thus, alpha iron oxide particles wereobtained. After the heat treatment, the particles were washed withwater, using an ultrasonic dispersing machine, so as to remove thenon-reacted product and residues. Then, the particles were furtherwashed with water, using the ultrasonic dispersing machine, and filteredand dried.

The resultant alpha iron oxide particles were subjected to an X-raydiffraction spectral analysis. As a result, a spectrum corresponding toan alpha hematite structure was clearly observed. Further, the shapes ofthe particles were observed with a transmission electron microscope. Asa result, they were found to be hexagonal plate particles having aparticle distribution of as narrow as 40 to 60 nm (average particlediameter: 50 nm). <<Components of Paint for Primer Layer>> (1) Platealumina particles 35 parts (average particle diameter: 50 nm) Plate ITOparticles 65 parts (average particle diameter: 40 nm) Stearic acid 2.0parts Vinyl chloride-hydroxypropyl acrylate copolymer 8.8 parts (—SO₃Nagroup content: 0.7 × 10⁻⁴ eq./g) Polyesterpolyurethane resin 4.4 parts(Tg: 40° C., —SO₃Na group content: 1 × 10⁻⁴ eq./g) Cyclohexanone 25parts Methyl ethyl ketone 40 parts Toluene 10 parts (2) Butyl stearate 1part Cyclohexanone 70 parts Methyl ethyl ketone 50 parts Toluene 20parts (3) Polyisocyanate 1.4 parts Cyclohexanone 10 parts Methyl ethylketone 15 parts Toluene 10 parts

<<Components of Paint for Magnetic Layer>> (1) Kneading step Ultrafinemagnetic powder (Nd—Fe—B) 100 parts (Nd/Fe: 2.4 atomic %, B/Fe: 9.1atomic %, σs: 132 A·m²/kg (132 emu/g), Hc: 192 kA/m (2,410 Oe), andparticle diameter (number-average particle diameter: hereinafterreferred to as average diameter): 25 nm) Vinyl chloride-hydroxypropylacrylate copolymer 14 parts (—SO₃Na group content: 0.7 × 10⁻⁴ eq./g)Polyesterpoyurethane resin (PU) 5 parts (—SO₃Na group content: 1.0 ×10⁻⁴ eq./g) Plate alumina particles 10 parts (average particle diameter:50 nm) Plate ITO particles 5 parts (average particle diameter: 40 nm)Methyl acid phosphate (MAP) 2 parts Tetrahydrofuran (THF) 20 partsMethyl ethyl ketone/cyclohexanone (MEK/A) 9 parts (2) Diluting stepPalmitic amide (PA) 1.5 parts n-Butyl stearate (SB) 1 part Methyl ethylketone/cyclohexanone (MEK/A) 350 parts (3) Compounding stepPolyisocyanate 1.5 parts Methyl ethyl ketone/cyclohexanone (MEK/A) 29parts

A paint for primer layer was prepared by kneading the components ofGroup (1) with a batch type kneader, adding the components of Group (2)to the mixture and stirring them, dispersing the mixed components with asand mill in a residence time of 60 minutes, and adding the componentsof Group (3), followed by stirring and filtering the mixture.

Separately, a paint for magnetic layer was prepared by previously mixingthe components for the kneading step (1) at a high rate and kneading themixture with a continuous twin screw kneader, adding the components forthe diluting step (2) and diluting the knead mixture with the continuoustwin screw kneader in at least 2 steps, dispersing the mixture with asand mill in a residence time of 45 minutes, and adding the componentsfor the compounding step (2), followed by stirring and filtering thedispersion.

The paint for primer layer was applied on a non-magnetic support (basefilm) made of an aromatic polyamide film (Mictron (trade name)manufactured by Toray, thickness of 3.3 μm, MD=11 GPa, MD/TD=0.70) sothat the primer layer could have a thickness of 0.6 μm after dried andcalendered. Then, the paint for magnetic layer was applied on the primerlayer by a wet-on-wet method so that the magnetic layer could have athickness of 0.06 μm after the magnetic paint layer had been oriented ina magnetic field, dried and calendered. After the orientation in themagnetic field, the magnetic layer was dried with a drier and farinfrared radiation to obtain a magnetic sheet. The orientation in themagnetic field was carried out by arranging N—N opposed magnets (5 kG)in front of the drier, and arranging, in the drier, two pairs of N—Nopposed magnets (5 kG) at an interval of 50 cm and at a position 75 cmbefore a position where the dryness of the layer was felt by one'sfinger. The coating rate was 100 m/min. <<Components of Paint forBackcoat Layer>> Plate ITO particles  80 parts (average particlediameter: 40 nm) Carbon black  10 parts (average particle diameter: 25nm) Plate non-magnetic iron oxide particles  10 parts (average particlediameter: 50 nm) Nitrocellulose  45 parts Polyurethane resin (containing—SO₃Na groups)  30 parts Cyclohexanone 260 parts Toluene 260 partsMethyl ethyl ketone 525 parts

The components of a paint for backcoat layer were dispersed with a sandmill in a residence time of 45 minutes and a polyisocyanate (15 parts)was added to the mixture to obtain a paint for backcoat layer. Afterfiltration, the paint was coated on a surface of the magnetic sheetopposite to the magnetic layer so that the resultant backcoat layercould have a thickness of 0.5 μm after dried and calendered, and then,the backcoat layer was dried to finish the magnetic sheet.

The magnetic sheet, thus obtained, was planished by seven-stagecalendering using metal rolls, at a temperature of 100° C. under alinear pressure of 200 kg/cm, and wound onto a core and aged at 70° C.for 72 hours. After that, the magnetic sheet was slit into tapes with awidth of ½ inch.

The components of a slitting machine (a machine for cutting the magneticsheet into magnetic tapes with a predetermined width) was adapted foruse, as follows. Tension cut rollers were arranged in the course of aweb route from a position where the magnetic sheet was drawn out, to agroup of slit cutters, and these tension cut rollers were made as a meshsuction type, in which porous metal was buried in the sucking portions.The machine was not provided with a mechanism for transmitting power tothe cutter-driving section, and thus, the cutter-driving section wasdirectly connected to a motor for performing direct driving.

A tape cut from the magnetic sheet was fed at a rate of 200 m/min. whilethe surface of the magnetic layer was being polished with a lapping tapeand a blade, and wiped to provide a magnetic tape. As the lapping tape,K10000 was used; as the blade, a hard blade was used; and Toraysee(trade name) manufactured by Toray was used for wiping the magneticlayer. The tape was treated under a feeding tension of 0.294 N. Themagnetic tape thus obtained was set in a cartridge to thereby provide amagnetic tape cartridge for use in a computer (hereinafter referred toas a computer tape).

FIG. 3 shows the computer tape thus obtained. As shown in FIG. 3, thiscomputer tape comprises a box-shaped casing body (1) constructed byjoining an upper casing section (1 a) to a lower casing section (1 b),and one reel (2) having the magnetic tape (3) wound thereon and arrangedin the casing body (1). An outlet (4) for drawing out the magnetic tapetherefrom is formed at one side of the front wall (6) of the casing body(1), and the outlet (4) is opened or closed by a slidable door (5). Atape-drawing member (7) is connected to an end portion for drawing outthe magnetic tape (3) so as to draw out the magnetic tape (3) wound onthe reel (2) from the casing for operation. Numeral 20 in FIG. 3 refersto a door spring for forcing the door (5) to freely close the outlet.

Example 2

A computer tape was made in the same manner as in Example 1, except thatmagnetic powder prepared by the following synthesis was used.

<<Synthesis of Ultrafine Magnetic Powder>>

Iron nitrate (III) (0.098 mol), cobalt nitrate (0.042 mol) and neodymiumnitrate (0.002 mol) were dissolved in water (200 cc). Separately fromthis aqueous nitrate solution, an aqueous sodium hydroxide solution wasprepared by dissolving sodium hydroxide (0.42 mol) in water (200 cc). Tothe aqueous nitrate solution, was added the aqueous sodium hydroxidesolution, and the mixture was stirred for 5 minutes to form a hydroxideof iron, cobalt and neodymium. This hydroxide was washed with water andfiltered to collect the hydroxide. To the hydroxide (containing water),were further added water (150 cc) and boric acid (0.1 mol) so as toagain disperse the hydroxide of iron, cobalt and neodymium in thisaqueous boric acid solution. The resulting dispersion was treated byheating at 90° C. for 2 hours, and washed with water to remove theexcessive boric acid. Then, the dispersion was dried at 60° C. for 4hours to obtain a boric acid-containing hydroxide which comprised iron,cobalt and neodymium.

This hydroxide was dehydrated by heating at 300° C. in an air for 2hours, and reduced by heating at 450° C. under a hydrogen stream for 4hours to obtain neodymium-iron-cobalt-boron magnetic powder. Then, themagnetic powder was cooled to room temperature under a hydrogen stream,and again heated to 650° C. under an atmosphere of a mixed gas ofnitrogen and oxygen. Then, the magnetic powder was treated forstabilization under the atmosphere of a mixed gas of nitrogen and oxygenfor 8 hours, and exposed to an air.

The resultant neodymium-iron-cobalt-boron magnetic powder was subjectedto a fluorescent X-ray analysis, and it was found that the content ofneodymium to iron was 1.9 atomic %; the content of cobalt to iron, 40.1atomic %; and the content of boron to iron, 7.5 atomic %. This magneticpowder was observed with a transmission electron microscope at amagnification of 250,000. As a result, the powder comprisedsubstantially spherical or ellipsoidal particles with an averageparticle diameter (number-average particle size) of 20 nm, as inExample 1. The saturation magnetization of the magnetic powder under theapplication of a magnetic field of 1,273.3 kA/m was found to be 157A.m²/kg (157 emu/g), and the coercive force thereof, 174.3 kA/m (2,190Oe).

Example 3

A computer tape was made in the same manner as in Example 1, except formagnetic powder. That is, the magnetic powder of Example 3 was preparedin the same manner as in Example 1, except that the temperature and thetime of the heat treatment in an air were changed to 120° C. and 4 hour,from 200° C. and 4 hours in Example 1, and that the particle diameterwas changed to 15 nm.

Example 4

A computer tape was made in the same manner as in Example 1, except thatultrafine plate magnetic powder (Ba-ferrite) (the average particlediameter (along the plane face)=25 nm; BET=67 m²/g; Hc=222 kA/m (2,790Oe); as =49.4 A.m²/kg (49.4 emu/g)) was used instead of the ultrafinegranular powder.

Example 5

A computer tape was made in the same manner as in Example 4, except thatthe amount of the plate alumina particles for the primer layer waschanged to 40 wt. parts from 10 wt. parts, and that the amount of theplate ITO particles was changed to 60 wt. parts from 90 wt. parts.

Example 6

A computer tape was made in the same manner as in Example 5, except thatthe thickness of the magnetic layer was changed to 0.04 μm from 0.06 μm,and that the thickness of the primer layer was changed to 0.45 μm from0.6 μm.

Example 7

A computer tape was made in the same manner as in Example 1, except that10 wt. parts of the plate alumina particles (average particle diameter:50 nm) and 5 wt. parts of the plate ITO particles (average particlediameter: 40 nm) in the paint for the magnetic layer were changed to 10wt. parts of granular alumina particles (average particle diameter: 80nm) and 2 wt. parts of carbon black particles (average particlediameter: 75 nm), respectively.

Example 8

A computer tape was made in the same manner as in Example 7, exceptthat, in the components of the paint for the backcoat layer, 80 wt.parts of the plate ITO particles (average particle diameter: 40 nm) waschanged to 0 wt. parts; 10 wt. parts of the carbon black particles(average particle diameter: 25 nm), to 80 wt. parts; 10 wt. parts of theplate iron oxide particles (average particle diameter: 50 nm), to 0 wt.part, and that 10 wt. parts of carbon black particles (average particlediameter: 0.35 μm) and 10 wt. parts of granular iron oxide particles(average particle diameter: 0.4 μm) were added to the paint for thebackcoat layer.

Example 9

A computer tape was made in the same manner as in Example 1, except thatthe components for the paint for the primer layer in Example 1 werechanged to the following. <<Components of Paint for Primer Layer>> (1)Needle iron oxide particles 68 parts (average particle diameter: 100 nm)Granular alumina particles 8 parts (average particle diameter: 80 nm)Carbon black particles 24 parts (average particle diameter: 25 nm)Stearic acid 2.0 parts Vinyl chloride-hydroxypropyl acrylate copolymer8.8 parts (—SO₃Na group content: 0.7 × 10⁻⁴ eq./g) Polyestepolyrurethaneresin 4.4 parts (Tg: 40° C., —SO₃Na group content: 1 × 10⁻⁴ eq./g)Cyclohexanone 25 parts Methyl ethyl ketone 40 parts Toluene 10 parts (2)Butyl stearate 1 part Cyclohexanone 70 parts Methyl ethyl ketone 50parts Toluene 20 parts (3) Polyisocyanate 1.4 parts Cyclohexanone 10parts Methyl ethyl ketone 15 parts Toluene 10 parts

Comparative Example 1

A computer tape was made in the same manner as in Example 1, except thatthe components of the kneading step (1) in <<Components of Paint forMagnetic Layer>> and the components of <<Paint for Primer Layer>> and<<Paint for Backcoat Layer>> were changed to the following. In thisregard, the magnetic powder was changed to magnetic powder comprisingneedle particles with a particle diameter (average axial length) of 100nm, and therefore, the thickness of the magnetic layer could not becontrolled to 0.06 μm, and it resulted in 0.11 μm. <<Components of Paintfor Magnetic Layer>> (1) Kneading step Needle ferromagnetic iron typemetal powder 100 parts (Co/Fe: 30 atomic %, Y/(Fe + Co): 3 atomic %,Al/(Fe + Co): 5 atomic %, σs: 145 A·m²/kg (145 emu/g), Hc: 187 kA/m(2,350 Oe), and average axial length: 100 nm, axial ratio: 7) Vinylchloride-hydroxypropyl acrylate copolymer  14 parts (—SO₃Na groupcontent: 0.7 × 10⁻⁴ eq./g) Polyesterpolyurethane resin (PU)  5 parts(—SO₃Na group content: 1.0 × 10⁻⁴ eq./g) Granular alumina particles  10parts (average particle diameter: 80 nm) Carbon balck particles  5 parts(average particle diameter: 75 nm) Methyl acid phosphate (MAP)  2 partsTetrahydrofuran (THF)  20 parts Methyl ethyl ketone/cyclohemanone(MEK/A)  9 parts

<<Components of Paint for Primer Layer>> (1) Needle iron oxide particles68 parts (average particle diameter: 100 nm) Granular alumina particles8 parts (average particle diameter: 80 nm) Carbon black particles 24parts (average particle diameter: 25 nm) Stearic acid 2.0 parts Vinylchloride-hydroxypropyl acrylate copolymer 8.8 parts (—SO₃Na groupcontent: 0.7 × 10⁻⁴ eq./g) Polyester-polyurethane resin 4.4 parts (Tg:40° C., —SO₃Na group content: 1 × 10⁻⁴ eq./g) Cyclohexanone 25 partsMethyl ethyl ketone 40 parts Toluene 10 parts (2) Butyl stearate 1 partCyclohexanone 70 parts Methyl ethyl ketone 50 parts Toluene 20 parts (3)Polyisocyanate 1.4 parts Cyclohexanone 10 parts Methyl ethyl ketone 15parts Toluene 10 parts

<<Components of Paint for Backcoat Layer>> Carbon black particles  80parts (average particle diameter: 25 nm) Carbon black particles  10parts (average particle diameter: 0.35 μm) Granular iron oxide particles 10 parts (average particle diameter: 50 nm) Nitrocellulose  45 partsPolyurethane resin (containing SO₃Na groups)  30 parts Cyclohexanone 260parts Toluene 260 parts Methyl ethyl ketone 525 parts

Comparative Example 2

A computer tape was made in the same manner as in Example 1, except thatthe components of <<Paint for Primer Layer>> and <<Paint for BackcoatLayer>> were changed to the following. <<Components of Paint for PrimerLayer>> (1) Needle iron oxide particles 68 parts (average particlediameter: 100 nm) Granular alumina particles 8 parts (average particlediameter: 80 nm) Carbon black particles 24 parts (average particlediameter: 25 nm) Stearic acid 2.0 parts Vinyl chloride-hydroxypropylacrylate copolymer 8.8 parts (—SO₃Na group content: 0.7 × 10⁻⁴ eq./g)Polyester-polyurethane resin 4.4 parts (Tg: 40° C., —SO₃Na groupcontent: 1 × 10⁻⁴ eq./g) Cyclohexanone 25 parts Methyl ethyl ketone 40parts Toluene 10 parts (2) Butyl stearate 1 part Cyclohexanone 70 partsMethyl ethyl ketone 50 parts Toluene 20 parts (3) Polyisocyanate 1.4parts Cyclohexanone 10 parts Methyl ethyl ketone 15 parts Toluene 10parts

<<Components of Paint for Backcoat Layer>> Carbon black particles  80parts (average particle diameter: 25 nm) Carbon black particles  10parts (average particle diameter: 0.35 μm) Granular iron oxide particles 10 parts (average particle diameter: 50 nm) Nitrocellulose  45 partsPolyurethane resin (containing SO₃Na groups)  30 parts Cyclohexanone 260parts Toluene 260 parts Methyl ethyl ketone 525 parts

Comparative Example 3

A computer tape was made in the same manner as in Comparative Example 2,except for the following. That is, the non-magnetic support (base film)was changed to an aromatic polyamide film (Mictron (trade name)manufactured by Toray; the thickness: 3.3 μm, MD=14 GPa, MD/TD=1.20)from the aromatic polyamide film (Mictron (trade name) manufactured byToray; the thickness: 3.3 μm, MD=11 GPa, MD/TD=0.70). In addition, thecomponents of the slitting machine were changed to the following: thetension cut roller was changed to a mesh suction roller in which aporous metal was embedded in the sucking portions of the suction type,and the direct cutter-driving by directly connecting the cutter-drivingsection to the motor without any mechanism for transmitting power to thecutter-driving section was changed to cutter-driving by usingconventional suction type cut rollers but not the tension cut rollers,and a rubber belt and a rubber coupling were used as the mechanism fortransmitting power to the cutter-driving section.

The methods for evaluation were as follows.

<Surface Roughness of Magnetic Layer>

The surface roughness of the magnetic layer was measured at a scanlength of 5 μm by a scanning type white light interference method, usingan universal three-dimensional surface structure analyzer, NewView 5000manufactured by ZYGO. The view field for measurement was 350 μm×260 μm.The center line average height of the surface roughness of the magneticlayer was measured as Ra.

<Output and Ratio of Output to Noises>

The electromagnetic conversion characteristics of the magnetic tape weremeasured using a drum tester. The drum tester was equipped with anelectromagnetic induction type head (track width: 25 μm, gap: 0.1 μm)and a MR head (8 μm) so that the induction type head was used forrecording, and the MR head, for reproducing. Both heads were arranged atdifferent positions relative to the rotary drum, and both heads wereoperated in the vertical direction to match their tracking with eachother. A proper length of the magnetic tape was drawn out from the reelin the cartridge and discarded. A further 60 cm length of the magnettape was drawn out and cut and processed into a tape with a width of 4mm, which was then wound onto the outer surface of the drum.

Output and noises were determined as follows. A rectangular wave with awavelength of 0.2 μm was written on the magnetic tape, using a functiongenerator, and the output from the MR head was read by the spectrumanalyzer. The value of a carrier with a wavelength of 0.2 μm was definedas an output C from the medium. The value of integration of a differenceof the subtraction at a certain wavelength of an output and system noiseof spectrum was used as a noise value N, when the rectangular wave witha wavelength of 0.2 μm was written on the magnetic tape. The ratio ofthe output to the noise was calculated as C/N. C and C/N are reported asrelative values in relation to the values of the tape of ComparativeExample 1.

<Coefficients of Temperature/Humidity Expansions of Tape>

Test pieces with a width of 12.65 mm and a length of 150 mm wereprepared from the magnetic sheet along the widthwise direction. Thetemperature expansion coefficient was determined from difference inlength between each of the test pieces exposed to atmospheres of 20° C.and 60% RH, and of 40° C. and 60% RH, respectively. The humidityexpansion coefficient was determined from difference in length betweeneach of the test pieces exposed to atmospheres of 20° C. and 30% RH, andof 20° C. and 70% RH, respectively.

<Amount of Edge Weave>

The amount of edge weave on either edge of the magnetic tape as thereference side for the running of the tape was continuously measuredalongside 50 m length of the tape, using a servo writer (running speed:5 m/sec.) equipped with an edge weave meter (Keyence). The resultantedge weave amount was subjected to Fourier analysis, and the amount ofan edge weave with a cycle of f (mm) was determined. Off-track wascaused by a component with a frequency V/f (1/s) of 50 (1/s) or more, oncondition that the tape-running speed was V (mm/s). Accordingly, theamount of edge weave referred to in the context of the present inventionis such that V/f(1/s) is not smaller than 60(1/s). In Examples andComparative Examples, the amounts of edge weaves were determined oncondition that the value of V/f (V=400 mm/s, f=65 mm) was equal to 61.5(1/s). The amount of off-track due to edge weave was determined byrunning the magnetic tape with a LTO driver.

<Amount of Off-Track Due to Changes in Temperature and Humidity>

The atmosphere at a temperature of 10° C. and a humidity of 10% RH waschanged to an atmosphere at a temperature of 29° C. and a humidity of80% RH. The maximal dislocation amount between the original position ofa track and a position of the same track which was dislocated due to theabove change in the atmosphere (the dislocation of the track position1,400 μm distant from the servo track) was determined from thetemperature expansion coefficient and the humidity expansion coefficientof the magnetic tape.

<Decrease in Output>

From the sum of the amount of off-track due to an edge weave and theamount of off-track due to changes in temperature and humidity, adecrease in output which occurred when the same apparatus was used forrecording/reproducing; and a decrease in output which occurred when anapparatus of which the position of a track was dislocated by 1.5 μm wasused were determined by calculation, on condition that the recordingtrack width was 12 μm, and the track width of the reproducing head, 10μm.

<Fluctuation in Output>

Variation in the thickness of the magnetic layer was evaluated asfollows. The magnetic tape was run at a rate of 2.54 m/sec. with a DLTdriver, and signals with a wavelength of 2 μm were recorded on a 50 mlength of the magnetic tape with a magnetic induction type recordinghead having recording tracks with a width of 76 μm. The signals werereproduced with a magnetoresistance reproducing head having tracks witha width of 38 μm. The reproducing output was read at a rate of 1,000positions/sec. (2.54 mm intervals in the tape lengthwise direction), andfluctuation in reproducing output was measured for evaluation. Thefluctuation rate of output in the tape lengthwise direction was definedby the average value of the following equation:Average fluctuation rate (MD indicates a value in the tape lengthwisedirection, which is the same also in Tables 1 to 3) (%)=(the absolutevalue of (output from each point average output)/average output)×100

The above operation was carried out on five tracks along the tapewidthwise direction, and the average value thereof was defined as therate of fluctuation in output in the tape lengthwise direction. Theamount of fluctuation in the tape widthwise direction was determined asfollows. The above data was determined from the output value from eachpoint in the tape lengthwise direction, by the following equation:the average fluctuation rate (TD indicates a value in the tape widthwisedirection, which means the same also in Tables 1 to 3) (%)=(the absolutevalue of (output from each track−an average of outputs from fivetracks)/an average of outputs from five tracks)×100.

An average of outputs from the respective points within a 50 m length ofthe tape was defined as an output fluctuation rate in the tape widthwisedirection.

Tables 1 to 3 show the results of the above evaluations and theconditions employed in Examples and Comparative Examples. TABLE 1 Ex. 1Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Magnetic Magnetic Element Nd—Fe—BNd—Fe—Co—B Nd—Fe—B Ba—Fe Ba—Fe Ba—Fe layer powder Shape Grain GrainGrain Plate Plate Plate Particle size 25 20 15 25 25 25 Filler Platealumina 10 10 10 10 10 10 (50 nm) Granular alumina (80 nm) CB (75 μm)Plate ITO (40 nm) 5 5 5 5 5 5 Primer Filler Plate alumina 10 10 10 10 4040 layer (50 nm) Needle iron oxide (100 nm) Granular alumina (80 nm) CB(25 nm) Plate ITO (40 nm) 90 90 90 90 60 60 BC Filler CB (25 nm) 10 1010 10 10 10 layer CB (0.35 μm) Granular iron oxide (0.4 μm) Plate ironoxide 10 10 10 10 10 10 (50 nm) Plate ITO (40 nm) 80 80 80 80 80 80Thickness of magnetic layer (μm) 0.06 0.06 0.06 0.06 0.06 0.04 Thicknessof primer layer (μm) 0.6 0.6 0.6 0.6 0.6 0.45 Thickness of support (μm)3.3 3.3 3.3 3.3 3.3 33 Thickness of BC layer (μm) 0.5 0.5 0.5 0.5 0.50.5 Total thickness (μm) 4.46 4.46 4.46 4.46 4.46 4.29 Young's modulusof base film (MD) 11 11 11 11 11 11 (GPa) Young's modulus of base film(MD/TD) 0.70 0.70 0.70 0.70 0.70 0.70 Slitting machine M + D M + D M + DM + D M + D M + D Surface roughness Ra (nm) 2.2 2.4 2.1 2.1 2.0 2.1 C(dB) 2.2 2.6 2.0 −3.6 −3.3 −3.6 C/N (dB) 8.4 9.5 10.5 7.9 8.2 8.1Temperature expansion coefficient 1.2 1.1 1.1 1.0 0.9 0.9 (TD) (×10⁻⁶/°C.) Humidity expansion coefficient (TD) 8.8 9.0 8.7 8.5 6.5 6.5 (×10⁻⁶/%RH) Amount of edge weave (μm) 0.6 0.6 0.6 0.6 0.5 0.5 Amount ofoff-track due to edge weave 0.08 0.08 0.08 0.08 0.07 0.07 (μm) Amount ofoff-track due to 0.89 0.91 0.88 0.86 0.66 0.66 temperature and humidityexpansions (μm) Total amount of off-track (μm) 0.97 0.99 0.96 0.94 0.730.73 Decrease in output (using the same 0 0 0 0 0 0 apparatus) (%)Decrease in output (using an 15 15 15 14 12 12 apparatus in which 1.5 μmdislocation of track occurs) (%) Average fluctuation rate of output 7.27.5 6.6 6.8 5.5 6.1 (MD) (%) Average fluctuation rate of output 7.0 6.95.2 5.3 5.1 5.4 (TD) (%)Slitting machine:M + D: mesh suction + direct drivingS + G: conventional suction + rubber belt, and driving by the use ofrubber coupling

TABLE 2 Ex. 7 Ex. 8 Ex. 9 Magnetic Magnetic Element Nd—Fe—B Nd—Fe—BNd—Fe—B layer powder Shape Grain Grain Grain Particle size 25 25 25Filler Plate alumina 10 (50 nm) Granular alumina 10 10 (80 nm) CB (75μm) 2 2 Plate ITO (40 nm) 5 Primer Filler Plate alumina 10 10 layer (50nm) Needle iron oxide 68 (100 nm) Granular alumina 8 (80 nm) CB (25 nm)24 Plate ITO (40 nm) 90 90 BC Filler CB (25 nm) 10 80 10 layer CB (0.35μm) 10 Granular iron 10 oxide (0.4 μm) Plate iron oxide 10 10 (50 nm)Plate ITO (40 nm) 80 80 Thickness of magnetic layer (μm) 0.06 0.06 0.06Thickness of primer layer (μm) 0.6 0.6 0.6 Thickness of support (μm) 3.33.3 3.3 Thickness of BC layer (μm) 0.5 0.5 0.5 Total thickness (μm) 4.464.46 4.46 Slitting machine M + D M + D M + D Young's modulus of basefilm (MD) 11 11 11 (GPa) Young's modulus of base film (MD/TD) 0.70 0.700.70 Surface roughness Ra (nm) 3.4 3.8 3.2 C (dB) 1.8 1.4 1.6 C/N (dB) 87.2 6.5 Temperature expansion coefficient 1.3 1.8 2.0 (TD) (×10⁻⁶/° C.)Humidity expansion coefficient (TD) 9.0 9.7 9.9 (×10⁻⁶/% RH) Amount ofedge weave (μm) 0.6 0.7 0.7 Amount of off-track due to edge weave 0.080.1 0.1 (μm) Amount of off-track due to 0.92 1.00 1.02 temperature andhumidity expansions (μm) Total amount of off-track (μm) 1.00 1.10 1.12Decrease in output (using the same 0 1.0 1.2 apparatus) (%) Decrease inoutput (using an 15 16 17 apparatus in which 1.5 μm dislocation of trackoccurs) (%) Average fluctuation rate of output 7.7 7.8 7.5 (MD) (%)Average fluctuation rate of output 7.0 7.5 6.8 (TD) (%)Slitting machine:M + D: mesh suction + direct drivingS + G: conventional suction + rubber belt, and driving by the use ofrubber coupling

TABLE 3 C. Ex. 1 C. Ex. 2 C. Ex. 3 Magnetic Magnetic Element Y—Fe—CoNd—Fe—B Nd—Fe—B layer powder Shape Particle size 100 25 25 Filler Platealumina 10 10 (50 nm) Granular alumina 10 (80 nm) CB (75 μm) 2 Plate ITO(40 nm) 5 5 Primer Filler Plate alumina layer (50 nm) Needle iron oxide68 68 68 (100 nm) Granular alumina 8 8 8 (80 nm) CB (25 nm) 24 24 24Plate ITO (40 nm) BC Filler CB (25 nm) 80 80 80 layer CB (0.35 μm) 10 1010 Granular iron 10 10 10 oxide (0.4 μm) Plate iron oxide (50 nm) PlateITO (40 nm) Thickness of magnetic layer (μm) 0.11 0.06 0.06 Thickness ofprimer layer (μm) 0.6 0.6 0.6 Thickness of support (μm) 3.3 3.3 3.3Thickness of BC layer (μm) 0.5 0.5 0.5 Total thickness (μm) 4.51 4.464.46 Slitting machine M + D M + D S + G Young's modulus of base film(MD) 11 11 14 (GPa) Young's modulus of base film (MD/TD) 0.70 0.70 1.2Surface roughness Ra (nm) 4.2 3.5 3.3 C (dB) 0 1.3 1.5 C/N (dB) 0 6.06.3 Temperature expansion coefficient 2.2 2.0 17.4 (TD) (×10⁻⁶/° C.)Humidity expansion coefficient (TD) 10.5 10.2 10.4 (×10⁻⁶/% RH) Amountof edge weave (μm) 0.8 0.8 2.5 Amount of off-track due to edge weave0.11 0.11 0.34 (μm) Amount of off-track due to 1.09 1.11 1.48temperature and humidity expansions (μm) Total amount of off-track (μm)1.20 1.22 1.82 Decrease in output (using the same 2.0 2.2 8.2 apparatus)(%) Decrease in output (using an 17 17 23 apparatus in which 1.5 μmdislocation of track occurs) (%) Average fluctuation rate of output 10.115.2 16.1 (MD) (%) Average fluctuation rate of output 9.4 13.1 13.4 (TD)(%)Slitting machine:M + D: mesh suction + direct drivingS + G: conventional suction + rubber belt, and driving by the use ofrubber coupling

As is apparent from Tables 1 to 3, the computer tapes (the magnetictapes) of Examples 1 to 9 of the present invention are excellent inelectromagnetic conversing properties and stability against changes intemperature and humidity, and are smaller in amounts of edge weaves, ascompared with the computer tapes of Comparative Examples 1 and 2.Therefore, the computer tapes of the present invention show less amountsof off-track even when the temperature and the humidity change. Inaddition, the computer tapes of the present invention show lessfluctuations in outputs both in the tape lengthwise direction and thetape widthwise direction: in other words, the magnetic layers of thepresent invention have less variation in thickness. In this regard, theamount of off-track is evaluated based on the calculation, on conditionthat the recording track width is 12 μm, and the track width of thereproducing head, 10 μm. The difference in amount of off-track betweenthe computer tapes of Examples and the computer tapes of ComparativeExamples will be more distinguishing, taken into consideration apossible recording track width of 10 μm or less which a computer tapewith a capacity of 1 TB is expected to have, and a possible track widthof 8 μm or less which a reproducing head is expected to have.

Effect of the Invention

According to the present invention, there are provided a magnetic tapeand a magnetic tape cartridge which show high reproducing outputs andhigh ratios of C/N, and are excellent in stability against changes intemperature and humidity. Thus, a backup tape for use in a computer,capable of corresponding to a recording capacity of, for example, 1 TBor more, can be realized.

1. A magnetic tape comprising a non-magnetic support, a magnetic layercontaining magnetic powder which is formed on one side of thenon-magnetic support, a primer layer containing non-magnetic powderwhich is formed between the non-magnetic support and the magnetic layer,and a backcoat layer containing non-magnetic powder which is formed onthe other side of the non-magnetic support, wherein the thickness of themagnetic layer is from 0.05 μm to 0.09 μm, and wherein the rate offluctuation in reproducing output in at least one of the tape lengthwisedirection and the tape widthwise direction is 8% or less when signalswith a wavelength of 2 μm are recorded on said magnetic tape with amagnetic induction type recording head having a recording track width of76 μm and are reproduced with a magnetoresistance type reproducing heada having track a width of 38 μm (the thickness of a magnetoresistancetype element: 0.05 μm).
 2. The magnetic tape according to claim 1,wherein said magnetic layer contains the magnetic powder which comprisesplate, granular or ellipsoidal magnetic particles with a number-averageparticle diameter of 5 to 50 nm, and wherein at least one of the primerlayer and the backcoat layer contains non-magnetic plate particles witha particle diameter of 10 to 100 nm.